Syväolento

Creature of the Deep (in-game name Syväolento) is a miniboss, found deep in the Lake. It is commonly referred to as Leviathan.

Behaviour
Syväolento shoots highly damaging piercing orbs, fired periodically in an octagonal pattern, and in your direction for every instance of damage. It slowly sways left and right, but never leaves the spot it spawned in.

It has 7500 health and is immune to projectile damage, while being highly resistant to all other types. Weakening Curses are recommended.

On death it will disappear, and permanently shift all water into smoke for the duration of the run. It will also drop a Full Health Regeneration, a Great Treasure Chest, and a red portal leading to the Teleport Room.

Tips

 * Poison Trail works well, taking it to 5%/375hp. Other methods of applying poison do not work.
 * Weakening Curses work, making projectile damage wands viable and thus making the fight relatively easy.
 * Spamming Summon Rocks at its location will cause the rocks to absorb the water, giving you an "air bubble" to fight in, e.g. if you want Stainless Armour to be active and to just tank the shots.
 * Homing modifiers will completely ignore it.
 * Going through the portal in NG+ runs will transport the player inside the Extremely Dense Rock walls, possibly suffocating them.