Colossal Stale Water Ooze

False Appearance: The Colossal Stale Water Ooze is indestinguishable from a mundane pool of stale water

Ambush: The Colossal Stale Water Ooze makes a Pseudopod attack against all creatures within its range before initiative, resuming initiative normaly after this attack.

Water Form: The Colossal Stale Water Ooze doesnt have a fixe shape, it can pass throw i inch wide gaps, otherwise it behaves like stale water, taking the shape of containers and flowing down normaly.

Stale Water Regeneration: The Colossal Stale Water Ooze regains 20 hit points at the start of its turn while in contact with sources of stale water.

Flood Waters: The Colossal Stale Water Ooze at initiative count 5 can reshape its form to ocupy a larger os smaller area within 30ft of it, depending on how many water volume it has available to flood its body across the area. 

Frozen Solid: Whenever the Colossal Stale Water Ooze takes 10 or more cold damage it partialy freezes reducing its speed by half until the end of its next turn.
If the cold damage dealt is 50 or more it is petrefied, restrained and paralyzed until the end of its next turn.

Cloged Drain: The Colossal Stale Water Ooze inhabits the larger reservour of the sewers of the ruined city of Myrmêt.

The drain is cloged, if it is cleaned up it returns to its function of draining the water and the Colossal Stale Water Ooze suffers the following penalties.

It takes 50 damage at the start of its turn (this damage cannot be reduced in any way), it has its regeneration desabled.

Furthermore it is forcefully pushed towards the drain reducing the area it has flooded (DM decision depending on the map).

Multiattack: The Colossal Stale Water Ooze makes one Pseudopod attack against all creatures within range.

Stale Water Pseudopod: Melee Weapon Attack +12 to hit 15ft reach, one target, for 4d10+7 bludgeoning damage + 2d6 acid damage.

A creature hit by this attack must succeed in a DC 20 Strenght or Dexterity saving throw being restrained by the stale dirty water on a failure, a creature restrained in this way is pushed underwater.

A creature restrained in this way may as an action attempt to break free by succeeding in a DC 20 Strenght saving throw, breaking free on a success.

Creatures with swiming speeds have advantage on this saving throws.