Dominaria Spoiler Digest - Who's Who and What's What from the Release Notes
Valakut, the Molten Pinnacle
Valakut, the Molten Pinnacle enters the battlefield tapped.
Whenever a Mountain enters the battlefield under your control, if you control at least five other Mountains, you may have Valakut, the Molten Pinnacle deal 3 damage to target creature or player.
T: Add R to your mana pool.
10/1/2009 Valakut, the Molten Pinnacle’s triggered ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless, at the time a Mountain enters the battlefield under your control, you control five or more Mountains other than that new one, and (2) the ability will do nothing if you control fewer than five Mountains other than that new one by the time it resolves.
10/1/2009 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.
10/1/2009 If multiple Mountains enter the battlefield 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 the battlefield at the same time as the one that caused it to trigger.
10/1/2009 You target a creature or player when the ability triggers. You decide whether to have Valakut deal damage to that creature or player as the ability resolves.