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So, I had a game with a friend today, and we had a discussion about Grey Knight Terminators as allies in a Black Templars army. My friend said that the Grey Knights benefit from the vows of the army (in this case ATCNMTO), while I maintained that, since it specifies that it affects "black templar units", it wouldn't affect GK termies, since they, obviously, are GK. As a funny twist, I was the one playing the BTs.

Anyway, who was correct, me or my friend?

Are Grey Knights a Black Templar unit?
No, they are a Grey Knight unit.

By hardcore RAW AACNMTO doesn't really do anything as no "Black Templars" units can be bought from the army list.

Anyway, this isn't similar to Vulkan that affects "friendly units", this affects "Black Templars units", hence not Grey Knights.

Steelmage has it spot on.