Adamantine Body. Any critical hit against the skeleton becomes a normal hit

Magic Resistance. The skeleton has advantage on saving throws against spells and other magical effects.

Turn Immunity. The skeleton is immune against effects that turn undead.

Flaming Body. Any creature that begins or ends its turn within 5 ft. of the skeleton must take 3 (1d6) fire damage.

Undying Nature. If the skeleton is destroyed, after 1d4 days a new body will from from the remains, and the skeleton will regain all of its hitpoints. 

Multiattack. The skeleton makes three slam attacks.

Slam. Melee weapon attack: +8 to hit, reach 5 ft., one target. Hit:  7 (1d10 + 5) bludgeoning damage plus 3 (1d6) fire damage.

Terrifying Howl. At the start of the skeleton's turn it lets loose a horrifying sound. Any creature within 30 ft. that isn't undead or deafened must make a DC 15 Wisdom saving throw or become frightened of the skeleton for the next minuet. Any creature that failed the saving throw can use their action to try again at the start of their next turn.

Fiery burst. Recharge (6). At the start of its turn the skeleton explodes, sending a wave of flames in every direction. Every creature within 10 ft. of the skeleton must make a DC 14 Dexterity saving throw. If the creature fails the saving throw it takes 15 (3d10) fire damage, or half as much on a successful saving throw. Creatures within 5 ft. must make a DC 18 Strength saving throw or be pushed back 5 ft.