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A note about Flash MTG In summary, flash is keyword ability that lets you cast a spell whenever you could cast an instant
Each of these can help turn the tide of a game, and is to be feared
Doctor's companion (You can have two commanders if the other is the Doctor.) The Doctor's companion ability allows you to have two commanders if one has the ability and the other is a legendary creature that is a Time Lord Doctor and has no other creature types
There is a way to template the ability so that it technically reads a little more blue (for example, instead of preventing the casting, it could allow it and then counter it) but it would just have been significantly wordier and would have had the same basic effect
Overlord of the Floodpits Remember Mulldrifter or Champion of Wits (or even Errant Ephemeron)