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Comparing the two powers I feel like trephination is terrible compared to shriek. Am I missing something?

Trephination
Type -Focussed witchfire
Range -18"
Effect -2D6+2 vs Ld
Result -Model takes that number of wounds, no armor or cover.

Psychic Shriek
Type -Witchfire
Range -18"
Effect -3D6 vs Ld
Result -Unit takes that number of wounds, no armor or cover.

They seem quite close at first glance but there are many minor things that sets Shriek way ahead of Trephination.
1. 3D6 can get you up to 18 vs 14 on 2d6+2. Trephination has a very slightly better low end of 4 vs 3.
2. Psychic Shriek is a primaris power so you can actually plan on having it for your game instead of hoping for Trephination.
3. Psychic Shriek is WC1 vs Trephination's WC2
4. Psychic Shriek affects the entire unit vs Trephination only affecting one model.

Now there is one upside to taking Trephination over Psychic Shriek, your DA librarian will get the bonus power for taking all Interromancy powers which has one of the best primaris powers.

Which brings me to my second point of this thread. I know it has way too many points of failure to be anything other than a long shot fluffy combo but, Mind Worm + Psychic Shriek. It was a popular combo using Weaken resolve (-1pt of Ld per psyker in the psyker battle squad) + psychic shriek to wreck MCs and terminators. Now the downside of mindworm is that it only affects the one model instead of the entire unit. But it is a str6 Ap2 shot that ignores cover. Also it reduces BS, WS, Init and Ld scores for the entire game. Hit almost any MC twice and it's forever WS, BS and Init 1, and Ld 4 or less forever. You don't even need to shriek it afterwards.

I will be taking a level two DA librarian with Mindworm and psychic shriek. Sure I will miss out on a random power but these two are both WC1 and extremely useful, plus I can always count on getting them. What are your thoughts on the Interromancy powers? Is mind worm as cool as I think it is or does the short range hinder it too much?

Trephination is indeed inferior in every way to Psychic Shriek.

Interromancy have two oyjer mildly good powers besides Mind Worm Though: the one that give you rage and the one that forces snapshots. Other then that there is overcosted and highly situational Mailsotrm of Misery (the one with mind shock SR).

The benefit of any Focused Witchfire is the ability to chose which Model suffers the effect, effectively the Psychic version of Precision Shots, if the Warp Charge cost is exceeded.

Except it is not how Focused Witchfire works. You just need to add one warp charge to select the model you wish to hit ot else your spell hits random model in targeted unit.

Trephination can snipe out models. I have removed a Company Commander because of this power for WL kill. It's not going to happen very often but it'll reliably take out Priests or Tau in general. If your target is a MC then hitting it with Mindworm first, followed by Trephination, will do serious damage.

Over all DAs seem to be about killing enemy characters to debuff enemy squads so they'll fall prey to all of the dark magic in our psychic discipline.

If you are not trying to exceed the Warp Cost, your not using Focused Witchfires correctly.