If Faceless Devourer leaves the battlefield before its first triggered ability has resolved, its last ability triggers. This ability does nothing when it resolves. Then its first ability resolves and exiles the target creature indefinitely.

Auras attached to the exiled creature will be put into their owners' graveyards. Equipment attached to the exiled creature will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist. Once the exiled creature returns, it's considered a new object with no relation to the object that it was.

If a token is exiled this way, it will cease to exist and won't return to the battlefield.

Three Faceless Devourers entering the battlefield at separate times with no other creatures with shadow on the battlefield will result in an infinite loop. The game ends in a draw unless a player somehow interrupts the loop.

Once a creature has been blocked, that creature remains blocked and will deal and be dealt combat damage even if it gains or loses shadow or if the blocking creature gains or loses shadow.

If an attacking creature has multiple evasion abilities, such as shadow and flying, a creature can block it only if that creature satisfies all of the appropriate evasion abilities.

Multiple instances of shadow on the same creature are redundant.