Tested Incidious Elements with Caress of Sivyana (resist mod 0, so should be the most affected spell along with Scitterpox -5).
[Fri Aug 11 20:55:50 2017] Your Caress of Sivyana did 1789 damage to a decadent elven guard.
[Fri Aug 11 20:55:56 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:56:02 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:56:08 2017] Your Caress of Sivyana did 896 damage to a decadent elven guard.
[Fri Aug 11 20:56:21 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:56:27 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:56:33 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:56:39 2017] Your Caress of Sivyana did 896 damage to a decadent elven guard.
[Fri Aug 11 20:57:30 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:57:36 2017] Your Caress of Sivyana did 1789 damage to a decadent elven guard.
[Fri Aug 11 20:57:42 2017] Your Caress of Sivyana did 1789 damage to a decadent elven guard.
[Fri Aug 11 20:57:42 2017] Your Caress of Sivyana did 896 damage to a decadent elven guard.
[Fri Aug 11 20:57:48 2017] Your Caress of Sivyana did 1789 damage to a decadent elven guard.
[Fri Aug 11 20:57:48 2017] Your Caress of Sivyana did 896 damage to a decadent elven guard.
[Fri Aug 11 20:57:48 2017] Your Caress of Sivyana did 1789 damage to a decadent elven guard.
[Fri Aug 11 20:57:54 2017] Your Caress of Sivyana did 896 damage to a decadent elven guard.
[Fri Aug 11 20:57:54 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:58:00 2017] Your Caress of Sivyana did 1794 damage to a decadent elven guard.
[Fri Aug 11 20:58:00 2017] Your Caress of Sivyana did 1789 damage to a decadent elven guard.
[Fri Aug 11 20:58:07 2017] Your Caress of Sivyana did 896 damage to a decadent elven guard.
[Fri Aug 11 20:58:13 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:58:19 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:58:25 2017] Your Caress of Sivyana did 896 damage to a decadent elven guard.
[Fri Aug 11 20:58:31 2017] Your Caress of Sivyana did 1793 damage to a decadent elven guard.
[Fri Aug 11 20:58:31 2017] Your Caress of Sivyana did 894 damage to a decadent elven guard.
[Fri Aug 11 20:58:37 2017] Your Caress of Sivyana did 1793 damage to a decadent elven guard.
[Fri Aug 11 20:58:37 2017] Your Caress of Sivyana did 1789 damage to a decadent elven guard.
[Fri Aug 11 20:58:43 2017] Your Caress of Sivyana did 898 damage to a decadent elven guard.
[Fri Aug 11 20:59:01 2017] Your Caress of Sivyana did 1789 damage to an elthannar wizard.
[Fri Aug 11 20:59:07 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 20:59:13 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 20:59:19 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 20:59:25 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 20:59:31 2017] Your Caress of Sivyana did 1789 damage to an elthannar wizard.
[Fri Aug 11 20:59:37 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 20:59:44 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 21:00:08 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:00:14 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:00:20 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 21:00:20 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:00:26 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 21:00:26 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 21:00:32 2017] Your Caress of Sivyana did 1793 damage to an elthannar wizard.
[Fri Aug 11 21:00:38 2017] Your Caress of Sivyana did 898 damage to an elthannar wizard.
[Fri Aug 11 21:00:51 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 21:00:57 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 21:01:15 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:01:21 2017] Your Caress of Sivyana did 1789 damage to an elthannar wizard.
[Fri Aug 11 21:01:21 2017] Your Caress of Sivyana did 1789 damage to an elthannar wizard.
[Fri Aug 11 21:01:27 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:01:27 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:01:33 2017] Your Caress of Sivyana did 1789 damage to an elthannar wizard.
[Fri Aug 11 21:01:33 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
[Fri Aug 11 21:01:45 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:01:51 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:01:57 2017] Your Caress of Sivyana did 894 damage to an elthannar wizard.
[Fri Aug 11 21:02:04 2017] Your Caress of Sivyana did 896 damage to an elthannar wizard.
Then I cast Embedding Darkness, Plague, Pyrocruor and then finally Caress of Sivyana (couldn't cast Archaic, they would die to fast).
This happent:
[Fri Aug 11 21:04:48 2017] Your Caress of Sivyana did 941 damage to a decadent elven guard.
[Fri Aug 11 21:05:36 2017] Your Caress of Sivyana did 943 damage to a decadent elven guard.
[Fri Aug 11 21:05:42 2017] Your Caress of Sivyana did 1887 damage to a decadent elven guard.
[Fri Aug 11 21:05:48 2017] Your Caress of Sivyana did 943 damage to a decadent elven guard.
[Fri Aug 11 21:06:49 2017] Your Caress of Sivyana did 941 damage to an elthannar wizard.
[Fri Aug 11 21:06:55 2017] Your Caress of Sivyana did 1882 damage to an elthannar wizard.
[Fri Aug 11 21:07:01 2017] Your Caress of Sivyana did 1882 damage to an elthannar wizard.
[Fri Aug 11 21:07:13 2017] Your Caress of Sivyana did 1887 damage to an elthannar wizard.
[Fri Aug 11 21:07:20 2017] Your Caress of Sivyana did 943 damage to an elthannar wizard.
[Fri Aug 11 21:10:13 2017] Your Caress of Sivyana did 910 damage to a decadent elven guard.
[Fri Aug 11 21:10:20 2017] Your Caress of Sivyana did 925 damage to a decadent elven guard.
[Fri Aug 11 21:10:26 2017] Your Caress of Sivyana did 925 damage to a decadent elven guard.
[Fri Aug 11 21:10:32 2017] Your Caress of Sivyana did 1885 damage to a decadent elven guard.
So thats not to bad.
Then I cast some Pyrocruor (-100 mod) on a respawn guard, just 1 dot, to see the difference... This is all mixed up, 1 dot and 3 and 4 dots on target...
[Fri Aug 11 21:03:04 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:03:10 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:03:17 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:03:23 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:03:29 2017] Your Pyrocruor did 887 damage to an elthannar wizard.
[Fri Aug 11 21:04:11 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:04:17 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:04:23 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:04:29 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:04:36 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:04:42 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:04:48 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:04:54 2017] Your Pyrocruor did 449 damage to a decadent elven guard.
[Fri Aug 11 21:05:18 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:05:24 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:05:30 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:05:36 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:05:42 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:06:43 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:06:49 2017] Your Pyrocruor did 887 damage to an elthannar wizard.
[Fri Aug 11 21:06:55 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:07:01 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:07:07 2017] Your Pyrocruor did 443 damage to an elthannar wizard.
[Fri Aug 11 21:07:13 2017] Your Pyrocruor did 887 damage to an elthannar wizard.
[Fri Aug 11 21:07:20 2017] Your Pyrocruor did 887 damage to an elthannar wizard.
[Fri Aug 11 21:07:50 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:07:56 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:08:02 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:08:08 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:08:14 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:08:20 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:08:27 2017] Your Pyrocruor did 443 damage to a decadent elven guard.
[Fri Aug 11 21:08:33 2017] Your Pyrocruor did 899 damage to a decadent elven guard.
[Fri Aug 11 21:08:45 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
[Fri Aug 11 21:08:51 2017] Your Pyrocruor did 887 damage to a decadent elven guard.
Nothing.
This is Embedding Darkness (0 mod), same, 1 dot, 2 dots, 3 dots, 4 dots, all mixed up.
[Fri Aug 11 21:02:58 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:03:04 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:03:10 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:03:17 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:03:23 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:03:29 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:04:05 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:04:11 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:04:17 2017] Your Embedding Darkness did 272 damage to a decadent elven guard.
[Fri Aug 11 21:04:23 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:04:29 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:04:36 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:04:42 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:04:48 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:04:54 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:05:12 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:05:18 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:05:24 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:05:30 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:05:36 2017] Your Embedding Darkness did 272 damage to a decadent elven guard.
[Fri Aug 11 21:05:42 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:05:48 2017] Your Embedding Darkness did 272 damage to a decadent elven guard.
[Fri Aug 11 21:06:19 2017] Your Embedding Darkness did 272 damage to an elthannar wizard.
[Fri Aug 11 21:06:25 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:06:25 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:06:31 2017] Your Embedding Darkness did 272 damage to an elthannar wizard.
[Fri Aug 11 21:06:31 2017] Your Embedding Darkness did 272 damage to an elthannar wizard.
[Fri Aug 11 21:06:37 2017] Your Embedding Darkness did 272 damage to an elthannar wizard.
[Fri Aug 11 21:06:37 2017] Your Embedding Darkness did 272 damage to an elthannar wizard.
[Fri Aug 11 21:06:43 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:06:43 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:06:49 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:06:49 2017] Your Embedding Darkness did 272 damage to an elthannar wizard.
[Fri Aug 11 21:06:55 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:06:55 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:07:01 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:07:01 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:07:07 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:07:13 2017] Your Embedding Darkness did 136 damage to an elthannar wizard.
[Fri Aug 11 21:07:20 2017] Your Embedding Darkness did 272 damage to an elthannar wizard.
[Fri Aug 11 21:09:37 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:09:43 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:09:49 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:09:55 2017] Your Embedding Darkness did 272 damage to a decadent elven guard.
[Fri Aug 11 21:10:01 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:10:07 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:10:13 2017] Your Embedding Darkness did 272 damage to a decadent elven guard.
[Fri Aug 11 21:10:20 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:10:26 2017] Your Embedding Darkness did 136 damage to a decadent elven guard.
[Fri Aug 11 21:10:32 2017] Your Embedding Darkness did 272 damage to a decadent elven guard.
Again nothing. I guess elthannarians have low MR.
So conclusion is, Insidious Elements deff. does something. Case closed.
Buuut... Because most necro spells has such high neg mod, it will only help a few of their spells, or if the mob is high resistant, yet not immune (or very highly resistant).
Not a fan, but nice to see that it does do something atleast