Xurid of a Thousand Echoes

Whenever you cast a card with flash, copy it. If
it's a permanent, return the copy to your hand.
Whenever you attack with a card with double
strike, create a copy of it that's tapped and
attacking. They both gain "If this creature would
have double strike, instead this has first strike".
Permanents you control with echo have "When
you pay this creature's echo cost, sacrifice it. If
you do, copy this creature".