Tiamat, Arisen Wyrm

Flying, indestructible
This spell costs {1} less to cast for each legendary
Dragon you control.
Tiamat has all abilities of other dragons on the battlefield.
If Tiamat would enter, if it wasn't cast or no mana
was spent to cast it, exile it instead.
Sacrifice five other differently named legendary
Elder Dragon creatures with mana value ten: You
win the game. This ability cannot be countered.