Hooded Hydra

If a face-down permanent leaves the battlefield, you must reveal it. You must also reveal all face-down spells and permanents you control if you leave the game or if the game ends.

Hooded Hydra's last ability isn't a triggered ability. It's a replacement ability that modifies how Hooded Hydra is turned face up. Players can't respond to Hooded Hydra being turned face up and having five +1/+1 counters put on it.

The face-down spell has no mana cost and has a mana value of 0. When you cast a face-down spell, put it on the stack face down so no other player knows what it is, and pay {3}. This is an alternative cost.

Use the number of +1/+1 counters that were on Hooded Hydra the last time it was on the battlefield to determine how many Snake tokens to create.

A permanent that turns face up or face down changes characteristics but is otherwise the same permanent. Spells and abilities that were targeting that permanent, as well as Auras and Equipment that were attached to the permanent, aren't affected.

Any time you have priority, you may turn the face-down creature face up by revealing what its morph cost is and paying that cost. This is a special action. It doesn't use the stack and can't be responded to. Only a face-down permanent can be turned face up this way; a face-down spell cannot.