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What would be the best discipline for a terminator level three who's unmarked? I'm thinking ectomancy and possibly a roll for psychic shriek

in what unit/army?

The other question is does the sorc have to be in terminator armour, because a bike will be better for him more often than terminator armour.

It also depends on who you're against, as rolling sinistrum can be a pretty good choice too as warp fate and death hex are superb but sometimes Heretek is the answer too.

9 times out of ten though, it's hard to go wrong with Biomancy or Telepathy.

Always take a spell familiar, its one of the only things CSM get uniquely that is better than loyalists.

Given the terminator armor I'm going to assume you going melee with him if that's the case then I would suggest Bio, or if your deep striking Pyro cause novas.

Without knowing more about your army, I second what Drasius said about varying based on what your opponent has. The main reason I bring sorcerers is so I have a way to adjust my list based on the enemy army. Don't just pick a discipline and always use the same one unless you're doing something like a deathstar that always needs invisibility.