Arena of Glory

If you gain control of another player's permanent until end of turn and exert it, and then that player regains control of it, it will untap during that player's untap step.

If mana generated with Arena of Glory's last ability is spent to pay any part of a creature spell's cost, including an alternative or additional cost, that creature spell (and the resulting creature) will gain haste until end of turn.

If mana generated with Arena of Glory's last ability is spent on a noncreature spell that becomes a creature later in the turn, that creature won't have haste.

You must already control a Mountain as Arena of Glory enters the battlefield for it to enter untapped. If it enters the battlefield at the same time as a Mountain when you control no other Mountains, it will enter tapped.