Beren, Kingbreaker

Split second
Flash, haste, double strike
When Beren, Kingbreaker enters the battlefield,
there is an additional combat phase, followed by
an additional mainphase in which Beren, Kingbreaker can only attack(This can occur on any player's turn and is treated as an interrupt.) Repeat
this process X times
Beren, Kingbreaker can only attack on extra
combat phases.
{t}Sacrifice Beren, Kingbreaker