Puddle Cave

Description
Near the start of mountain range 2, There is a cave with a puddle in it. the mouth of the cave is worn smooth, past the mouth of the cave reveals an abundance of stalagmites and stalagmites, some of them very nearly grown together. as players advance farther into the cave the ceiling curves up and away from them, as they enter a giant basin, where water drips echo, as a nearly constant string of plinks. the rocks under foot are slippery, with tiny rivulets of water.

In the middle of the chamber is a large puddle nearly 15 feet across. near the middle, but not quite there there is a single rock, with a single magical rune glowing on it, ever so faintly(dc13 perception to casually notice) if there is a light source, otherwise it is the only light in the room.

If a medium creature steps on it,  12 seconds or so later, there is a cracking noise, and a terrible scraping, as a section of wall roughly 90 degrees front he entrance cracks and splits open revealing a short tunnel. any player standing next to the wall makes a dc 10 dex saving though or take 1d4 dmg, from rock shards splintering and peppering them.

The short tunnel of maybe 10 feet reveals a heavily rusted door. There is a faint inscription in the right side of the door frame. runes written in a vertical line read "Valin's key opens Valin's door" below that in a slightly different script, more jagged and with harder lines, " but Stenraf's key opens any door my brother". The door could be pick open, with a dc 20 check, or forced open with a DC 25 check, or a key may be used to open it.

1 hour after opening the wall the wall closes, and seals anyone inside. The players should be warned of this by passingly mentioning a sense of stale air in the hall, or mentioning the bones of several animals curled together near the wall or door. Generally try to communicate to the players that it is a trap. There is another glow plate  on the other side for the door which opens the wall. if the wall starts to close is sounds like a giant breath being let out, light a giant man putting down a heavy load. it takes 20 seconds for the wall to close, or about 5 minutes of players interacting around. near the 10 second mark a passive perception of 10 would be enough to notice the wall lowering back into place, leaving the players with about rounds to escape.

Creatures
2d4 giant goats might be in the entry way

or

1d4+1 hell hounds + 1 Cerberus may approach the cave at night

in the main cavern player might run into

1d4 gray oozlings

1d4 swarming oozes

1 Aura ooze

Dungeon(Undiscovered)
Directly above the puddle there is an opening in a large stalactite. which leads to a tunnel, and a dungeon, a DC 17 Perception check to notice the cut out of the stalactite.

DC 15 investigation check to notice that there are substantially more drips of water coming from the hidden tunnel opening.

The Dungeon is TBA

The Rewards of the dungeon are:

Gems(5,000GP), eroded dwarven poetry on stone slabs as light as feathers, +2 warpick, suit of +1 full plate and the 'Mountain Brush Key'