Myth Unbound’s cost reduction applies after you apply the commander tax. In effect, this reduces the commander tax to .

Myth Unbound’s cost reduction applies even if you’re casting your commander from a zone other than the command zone.

If Myth Unbound leaves the battlefield at the same time as your commander is put into the command zone, its last ability won’t trigger.

If another player is somehow able to cast your commander, Myth Unbound’s first ability applies to that player’s spell.