Karrthus, Tyrant of Jund

Flying, haste
When Karrthus enters, gain control of all Dragons, then untap all Dragons.
Other Dragon creatures you control have haste.

Secret Lair Drop

Double Masters

Flying, haste
When Karrthus, Tyrant of Jund enters the battlefield, gain control of all Dragons, then untap all Dragons.
Other Dragon creatures you control have haste.

The List

Commander's Arsenal Oversized

Alara Reborn

Flying, haste
When Karrthus, Tyrant of Jund comes into play, gain control of all Dragons, then untap all Dragons.
Other Dragon creatures you control have haste.

The control-change effect of Karrthus's triggered ability lasts indefinitely. It doesn't wear off during the cleanup step, and it doesn't expire if Karrthus leaves the battlefield. In a multiplayer game, it does expire if you leave the game.

When the triggered ability resolves, you'll untap all Dragons, including the ones you already control.