Ostroth, Loremaster at Arms

Vigilance, Eminence—
Legendary creatures cost {1} less to cast and are
the ally type, Commander creatures cost {1} less
for each time they've been cast this game as long
as Ostroth is untapped or in the command zone
{t}:Prevent the next X damage that would
be dealt to target ally or player this turn where
X is the number of legendary creatures
on the battlefield