The vampire who ceases to block is still considered to have blocked the action (e.g. they can play Cats' Guidance).
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Coterie Tactics
Choose another ready unlocked vampire you control of the same sect as this reacting vampire. These two vampires attempt to block, using the sum of their intercept to see if the block succeeds. If successful, both lock and then you choose one to be the blocking vampire (and the other ceases to be blocking).
Rulings
Can be used by a vampire who is already attempting to block.
Cannot be used if the vampire cannot attempt to block the action (eg. Daring the Dawn).
Does not ignore the normal prey, predator or target restrictions.