Poison Resistance. Most of the Night Walkers attacks are poison, so it is immune to being poisoned.

Decapitated. If the Night Walker is decapitated, it still acts as if it has a head and searches for it. It's body grows many hands, that when it touches something they getting poisoned for a week. 

Spellcasting. The Night Walker is a 20-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 14, +8 to hit with spell attacks). The Night Walker has following Sorcerer spells prepared:
Cantrips (at will): Acid Splash, Poison Spray 
1st level (2 slots): Fog Cloud, Thunderwave

2nd level (1 slot): Scorching Ray

3rd level (1 slot): Fear

Multi-attack. The forest spirit makes 6 attacks with it's hands.

Elemental Hands. Elemental  Attack: +10 to hit, reach 400 ft., 10 targets. Hit: 25 (2d10 + 10) poison damage. When the targets are touched they must make a DC 15 Constitution saving throw. If missed the DM must roll a d6. 1-5 the targets take given damage and 6 the targets get all the life sucked out of them and are paralyzed,but not dead. If the targets succeed the saving throw they take half damage. 

Search. If the Night Walker's head is decapitated it grows hands where it's head would be and starts searching for it's head.