Multiattack: Boneshard Skeleton can make two attacks on its turn. 

Boneblade. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) slashing damage plus 3 (1d6) necrotic damage.

Boneshards. (Recharge 5-6) The Boneshard Skeleton slams on the ground spraying bone shards into the air in a 15ft cone in front of it. Creatures in the cone must make a DC 14 Dexterity saving throw or take 10 (4d4+3) piercing damage. Creatures hit with bone shards must make a DC 13 Constitution saving throw. On a failed save creatures take 1d8 necrotic damage at the beginning of its turn. Creatures can make another constitution save at the end of its turns to end the effects.

Burst: When Boneshard Skeleton is reduced to below half its maximum hit points it bursts a bone shard spray in a 15ft radius around itself. Creatures within the 15ft radius must make a DC 13 Dexterity saving throw. On a fail creatures take 2d6 + 6 necrotic damage. On a success creatures take half damage. Boneshard Skeleton does a final burst when its hit points reach 0.