Spellcasting.

The skull lasher is a 3rd-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 13, +5 to hit with spell attacks). It has the following wizard spells prepared:

Cantrips (at will): mage hand, message, prestidigitation
1st level (4 slots): detect magic, protection from evil and good, ray of sickness (see "Actions" below), shield
2nd level (2 slots): darkness, misty step

Actions

Multiattack.

The skull lasher makes two attacks with its flail.

Iron Skull Flail.

Melee Weapon Attack: +2 to hit, reach 5 ft., one target. Hit: 4 (1d8) bludgeoning damage plus 7 (2d6) necrotic damage, and the target has disadvantage on all saving throws until the end of the skull lasher's next turn.

Ray of Sickness (1st-Level Spell; Requires a Spell Slot).

Ranged Spell Attack: +5 to hit, range 60 ft., one creature. Hit: 9 (2d8) poison damage, and the target must succeed on a DC 13 Constitution saving throw or be poisoned until the end of the skull lasher's next turn. If the skull lasher casts this spell using a spell slot of 2nd level or higher, the damage increases by 1d8 for each slot level above 1st.

Skull Lasher of Myrkul