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So in the codex where they go in detail of each aspect, its states that the warlocks must choose one power, while under the army list section under the farseer entry it states that the warlock "may" take a power.
So do they have to choose a power or not?
And one more, do all the warlocks in a group have to have the same spell or can they differ?
Right I'm not positive on the first question, but you can indeed mix and match powers in a warlock unit.
Fortes Fortuna Adiuvat
Cool, now I can broaden my horizons with my army
This was also the case in the 3rd edition codex, but then they could choose not to buy witchblades also. (Meat shield warlocks at 13 points was nuts!)
Warhammer Fantasy: Warriors of Chaos, High Elves
Warhammer 40k: Eldar, Space Marines, Orks