Ixor, Champion of Amonkhet

You may exert Ixor, Champion of Amonkhet as it attacks. (An exerted creature won't untap during your next untap step.)
At the beginning of your postcombat main phase, add X in any combination of {r}, {g} or {w}, where X is the number of creatures you exerted this turn.
{r}{g}{w}: untap target creature that attacked this turn.