Bloodwood Goblin Hetman

The power of the Mask. The goblin hetman makes all saving throws at advantage.

Madness of Magic. Whenever casting a spell targetting the goblin hetman, roll a d8 . On a roll of 1, the spell rebounds on its caster. The hetman suffurs no damage or effects from the spell, and thes are intead resolved on the caster.

Spellcasting. The Goblin Hetman is a 4th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 11, +3 to hit with spell attacks). It has the following spells prepared:
Cantrips (at will): Guidance, Resistance, Thaumaturgy
1st level (4 slots): Bless, Command, Fog Cloud
2nd level (3 slots): Blur, Heat Metal

Multiattack The goblin hetman makes two Bone Axe attacks

Bone Axe. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (1d10+4) slashing damage. 

Summoning Circle. The hetman's summoning circle is the centre of his power, and he uses it to wield the terribble magical power required to call forth strange magical beings that serve him as living weapons. The circle is 15 ft. in diameter and inscribed with a mixture of standard magical formulae and esoteric sigils of the hetman's own invention. Within the summonig circle the hetman is able to focus his power more intensely and he becomes even more dangerous.

The summoning circle, while active, grants the hetman +1 AC, resistance to damage from spells, and allows him to make an additional bonus action per turn while within the summoning circle, Aswell as a bonus action spell attack dealing (1d6) force damage.

Any player charachter can attempt to enter the circle at any time, though doing so has consequences if it has not yet been deactivated. Initially, the summoning circle crackles with energy, and passing into it requires a player character to make a DC 14 Constitution saving throw. A failure results in the player charachter being flung backwards 10 ft. and taking  (1d8) force damage. if the player character succeeds on the save, they may enter the summoning circle. Player characters take (1d8) force damage for each turn they are within the circle while its still active. 

Attacks can be aimed at the summoning circle (AC14, 25 hit points), but it can only be damaged by magical and melee attacks.

Once the summoning circle has been reduced to 0 hit points, it is deactivated and can be entered freely without risk of harming the player characters.