Multiattack

Koschei makes two longsword attacks and one drain life attack.

Longsword

Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) slashing damage or 11 (1d10 + 6) slashing damage if used in two hands plus 14 (4d6) necrotic damage.

Drain Life

Melee Spell Attack: +11 to hit, reach 5 ft., one target. Hit: 20 (4d6 + 6) necrotic damage. The target must succeed on a DC 19 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken from this attack, and Koschei regains an equal number of hit points. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.