Actually the pool is only about 5 feet deep
Zal can tell that the space beyond the door is another large space not a corridor but again the darkness prevents him seeing more than 10 feet ahead..he opens the door to find that the space is in fact the castle dungeon.
There is a large nest like pile of detritus in the middle of the room in which sits a Large and extremely ugly Humanoid..
"Ere you can't come in 'ere" it rumbles in bad common
Surprise round with a difference..you get two actions apiece
At seeing the large, appearance-challenged humanoid, Ulf gives another roar of battle, charging forward and swinging his axe at the thing with little regard for who or what it is.
Charge Attack: 1d20 + 9 + 2 - 1 ⇒ (2) + 9 + 2 - 1 = 12
Damage: 1d12 + 8 + 3 ⇒ (5) + 8 + 3 = 16