(2019-11-12) You must cast the revealed card while Underdark Beholders triggered ability is resolving
You play it because it answers almost everything
Its cost is relatively high, but it doesnt require much to equip, its typing interacts with Kaito, Bane of Nightmares, and Dimir Tempo lists currently use plenty of cheap creatures with flying, making it a way to turn any of them into an efficient clock and hand filtering, not to mention potential instant-wins with Dig Through Time and the like
If the opponent Kaalia attacked is no longer in the game when its ability resolves, you may put an Angel, Demon, or Dragon creature card onto the battlefield tapped, but it won't be attacking anyone and it won't be an attacking creature
Get your Reserved List cards graded by a grading company, like Becketts, to ensure that your cards hold their value