Maybe I'm being thick but just to clarify the use of daemons with black legion:

If I buy a greater daemon for my army, does my summoning pool automatically get the +8 summoning points added to my daemon pool needed to bring it out during the game? (obviously I would have to roll to get them on the actual turn.)

If so, can those 8 summoning points be used to summon lesser daemons instead if at some point in the game I decide that it would make better tactical sense dependant on the situation?

Or should there be 2 separate and distinct summoning pools for lesser and greater daemons?

Really what I'm probably asking is: Is purchasing a greater daemon simply giving you the option of having it on the board and the pool points to realise that option, or must those points be earmarked to be spent on the greater daemon during the game?

Makes sense. So always have 2 daemon pools then, greater and lesser.

Effectively, except you can pick from both at the same time.

Hypothetical question: can the daemon pool be a BTS?

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I suppose theoretically. However, since all summoned units leave play in the End Phase, it would automatically give that condition to your opponent, similar to off-board Necrons. I think.

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SG

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No BTS. It's not a formation as such and follows none of the rules for one. It may not be listed separately in every chaos list, but the convention has become to list the pool separately from other formations for this reason.