They can't be the targets of spells or abilities, their static abilities have no effect on the game, their triggered abilities can't trigger, they can't attack or block, and so on
There are several hidden mechanics never explained in the game which is only found explained in patch notes from the developers, which is why we are taking a deep dive into this today and are trying to gather all the information in one place
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The attacking creature stays alive
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Specifically, there are a number of legendary creatures with the Doctor's Companion ability, like Rose Tyler and in that case you can have two commanders, a Doctor and a Doctor's Companion