Asmodeus's spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). Asmodeus can innately cast the following spells at any level, requiring no components except for materials with a gold cost:

At will: Antimagic Field, Ashardalon's Stride, Bane, Banishment, Blindness/Deafness, Chain Lightning, Command, Contingency, Control Flames, Control Weather, Counterspell, Delayed Blast Fireball, Demiplane, Disintegrate, Dispel Evil and Good, Dispel Magic, Dissonant Whispers, Divine Word, Dominate Monster, Earthbind, Fabricate, Fear, Feather Fall, Fireball, Fire Storm, Fly, Forcecage, Greater Invisibility, Haste, Heat Metal, Hellish Rebuke, Hold Monster, Hold Person, Immolation, Incendiary Cloud, Infernal Calling, Lightning Bolt, Locate Creature, Locate Object, Maddening Darkness, Misty Step, Power Word: Kill, Scrying, Sending, Sequester, Shield, Silence, Slow, Suggestion, Summon Fiend (Devil Only), Telekinesis, Teleport, Time Stop, Wall of Fire, Wristpocket

1/day: Feeblemind, Gate, Meteor Swarm, Planar Binding, Storm of Vengeance, Time Ravage

3/day: Bestow Curse, Heal, Modify Memory, Phantasmal Force, Prismatic Spray, Psychic Scream, Ravenous Void, Reality Break, Reverse Gravity, Sleep, True Polymorph

Special Equipment. Asmodeus is equipped with the Ruby Rod of Asmodeus.   Lord of Lies. Asmodeus has advantage on all Charisma ability checks and saving throws. He also treats any roll of 9 or lower as a 10. He is immune to magic that allows other Creatures to read his thoughts, determine whether he is lying, know his alignment, or know his Creature Type. Creatures can telepathically communicate with him only if he allows it. Additionally, no matter what you say, magic that would determine if you are telling the truth indicates that you are being truthful.   Divine Form. Asmodeus regains 30 hit points at the start of each of his turns. Additionally he does not require food, drink, sleep or air. At the end of every hour Asmodeus can regain all uses of a spell of his choice.   Shifting Form. Asmodeus can as a bonus acting take on the appearance of any creature of his size or return to his true appearance. He can also change size while in his true form, either by increasing his size category or reducing it by 1 to a minimum of Small. If his size increases his height doubles, his reach with melee attacks increases by 5ft, his range with range or thrown weapons is doubled and his movement increases by 10ft. The opposite happens if he reduces his size to minimum of the the stats presented in this stat block, except for height. If Asmodeus increases his size while already Gargantuan all other effects happen but his size category doesn't change, if Asmodeus then reduces his size, the other effects happen but his size category is only decreased if he is at his standard Gargantuan height.   Weapons of Darkness. Asmodeus can create any +3 magic weapon at will, these function as normal except they deal 9d8 damage of the weapons type and can have a modified reach or range as per his Shifting Form ability. Additionally any time he hits with any attack he deals an additional 6d12 damage of a type from Asmodeus's choosing from fire, cold, or of a type dealt by the attack, this is included in the attack.