Ethereal Sight: Rusalka can see 60 ft. into the Ethereal Plane when she is on The Material Plane, and vice versa.

Seaglass Skin: Rusalka is fully obscured underwater. 

Incorporeal Movement: Rusalka can move through other Creatures and Objects as if they were Difficult Terrain. She takes 5 (1d10) force damage if she ends her turn inside an object.

Spellcasting. Rusalka is a 17th-level spellcaster. Her spellcasting ability is Intelligence (spell save DC 18, +12 to hit with spell attacks). Rusalka has following spells prepared:
Cantrips (at will): Eldritch Blast, Lightning Lure

1st level: (7 slots): Cause Fear, Charm Person, Puppet, (Puppet as represented by Unearthed Arcana) Sleep, Frost Fingers, Command.

3rd Level: (5 slots): Bestow Curse, Counterspell, Haste, Slow, Vampiric Touch, Life Transference

5th Level: (3 slots): Modify Memory, Dominate Person, Negative Energy Flood.

6th Level: (2 slots) Chain Lightning, Disintegrate, Circle of Death

7th Level (2 slots): Regenerate, Tether Essence

8th Level (1 slot) Antimagic Field, Maddening Darkness

Sing: Rusalka wails, making any creatures casting spells with verbal components for the next round roll a DC 7 wisdom save or lose their place and become unable to cast.

Spectral Claws: +5 to hit, reach 5 ft., 1 target. Hit: 6 (1d6 + 2) slashing damage. 

 Ethereal Leap: Rusalka enters the Ethereal plane.

 Vicious Return: Rusalka enters the Material Plane, gaining advantage on attacks for the turn.