Valakut, the Molten Pinnacle

If a Mountain you control leaves the battlefield between the time Valakut's second ability triggers and the time it resolves, be aware if that was the Mountain that caused Valakut's ability to trigger or not. If it was, Valakut's count isn't affected; if it wasn't, Valakut's count goes down by one.

You target a permanent or player when the ability triggers. You decide whether to have Valakut deal damage to that permanent or player as the ability resolves.

If multiple Mountains enter under your control at the same time, Valakut's second ability could trigger that many times. Each ability takes into consideration the other Mountains that entered at the same time as the one that caused it to trigger.