If I attack with {Lorthos, the Tide Maker}, use his mana ability to tap eight permanents, and also attack with a creature that's enchanted with {Bear Umbra}, do the lands I tapped for Lorthos untap?
When Lorthos attacks, both abilities are triggered at the same time. Since you control both abilities, you can choose the order in which they are put on the stack.
If Lorthos' trigger goes first, you tap {8} for the ability and it resolves. Then the umbra ability resolves and you untap all your lands.
RESOLVED
But of course! What would be preventing them from untapping?
Nothing, I guess. I was just wondering, cause a friend and I were discussing it. Glad to have it resolved.
It is good that you can choose how the stack goes! ^_^