Spellcasting. Lady Van Tassel is an 10th-level spellcaster. Her spellcasting ability is Intelligence (spell save DC 15, +7 to hit with spell attacks). She has the following spells prepared:
Cantrips (at will): eldritch blast, mage hand, minor illusion, thaumaturgy
1st level (4 spell slots): command, detect magic, mage armor
2nd level (3 spell slots): blindness/deafness, enthrall, scorching ray
3rd level (3 spell slots): bestow curse, counterspell, fireball, remove curse
4th level (3 spell slots): banishment, summon greater demon
5th level (2 spell slots): danse macabre

Magical Resistance. Lady Van Tassel has advantage on saving throws against spells and other magical effects.
Summon Headless Horseman. Lady Van Tassel is able to summon the Headless Horseman from the pits of hell as long as she has his skull is with her and she has given him a target or targets. The Headless Horseman is summoned 1 mile away from Lady Van Tassel's current location and will follow her orders given while she summoned him. The Headless Horseman will not leave the material plane until it has fulfilled the orders Lady Van Tassel has given him.