I have fun modifying official monsters, giving to them unique powers.
In the wonderful AD&D 2nd edition sourcebook "Van Richten's Guide to Liches", there's a list of 20 Lich's additional powers.
I've converted them in 20 new Lich's features, actions or reactions for D&D 5e.
You can rule that actions can also be legendary or lair actions.
Choose from 1 to 3 to be added to your lich, or to other undead creatures, making it a truly unique opponent for your players.
Some are more powerful than the others, so when you choose them modify accordingly the lich’s CR.
Animate Dead by Will (Action). The lich can cause zombies and skeletons to rise with its mere will. Such creatures have advantage on saving throws against the cleric's Channel Divinity: Turn Undead as long as they are within 200 feet of the lich. The lich may rise as many creatures as many corpses are available.
Bone Command (Action). The lich can animate bone and shape it at will. It chooses one of the following effects.
Bone Barrier. The lich call up splinters of bones from anywhere bones are presente and animate them into a protective barrier as per the blade barrier spell. Thre lich doesn't need to mantain concentration on this effect.
Create Structure. The lich form bones into any structure it desires, the only limit being the amoount of bone available. Common structures that can be created are huts, arches, bridges and so forth. The DM determines the structure's integrity and size, according to the scenario and the type and amount of bones available. The structure lasts until destroyed or the lich dismisses it with a bonus action. The DM also determines the hp and the resistance of the bone structure. As a rule of thumb, generally, on a successful Strength (Athletics) DC 15 check a creature is able to breach a bone wall.
Chilling Wind (Action). The lich blows a freezing blast of wind. Each creature in a 60-foot cone take 4d10 cold damage and must succeed on a DC 18 Wisdom saving throw or be frightened for 1 minute. The frightened target can repeat the saving throw at the end of oeach of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to the frightened condition caused by the lich's wind for the next 24 hours.
Coldfire (Action). Ranged Spell Attack: +12 to hit, reach 60 ft., one creature. Hit: 10 (3d6) cold damage. The target must succeed on a DC 18 Constitution saving throw or it takes additional 10 (3d6) necrotic damage.
Conjure Undead Legion (1/day) (Action). The lich summons undead creatures that appear in 1d4 rounds in unoccupied spaces that it can see within 120 feet. It can summon
- 12 undead of CR 1 or lower
- 6 undead of CR 3 or lower
- 3 undead of CR 5 or lower
- 1 undead of CR 9 or lower
They obey to the lich's telepathic commands, acting in its turn, and they disappear when they drop to 0 hp or are dismissed by the lich as a bonus action.
Doom Gaze (Action). The lich fixes its gaze on one creature it can see within 10 feet of it. If the creature it chooses has 50 hp or fewer, it dies. Otherwise it takes 10 (3d6) psychic damage.
Dream Manipulation (Feature). The lich, if it knows the exact location of a living humanoid creature, can infiltrate that creature's mind and manipulate its dreams, turning them into nightmares of the lich's design. The creature is aware that its dreams are tampered with, but it's unable to prevent this. The creature can't benefit of the effect of a long rest. If the lich manipulate a creature's dreams for a week, that creature suffers the effects of a long term madness randomly chosen as described in the DMG.
Fool's Feast (Feature). The lich taints the food within 5 feet of it. A creature that eats the tainted food must succeed on a DC 18 Constitution saving throw or be poisoned and die in 1 minute. A poisoned creature can be cured by protection from poison, greater or lesser restoration.
Grasp of Death (Action). Melee Spell Attack: +12 to hit, reach 5 ft., one creature. Hit: 10 (3d6) necrotic damage. If the target has 50 hp or fewer, it must succeed on a DC 18 Constitution saving throw or die.
Grasp of Enfeeblement (Action). Melee Spell Attack: +12 to hit, reach 5 ft., one creature. Hit: 10 (3d6) cold damage. On a hit, the target deals only half damage with weapons attacks thtat use Strength and it has disadvantage on Strength checks for 1 minute.At the end of eacht of its turns, the target can make a DC 18 Constitution saving throw. On a success, the effect ends.
Ignore Metal (1/day) (Bonus Action). As a bonus action for 1 minute, the lich ignore the existence of metal with its physical body by magically putting itself slightly out of dimensional phase. It gains immunity from magical weapons made of metal, but it still takes damage equal to the magical pluses of the weapon (e.g. it takes 2 damage from a magical sword +2 made of steel) and it can normally move through metal objects such as an iron door.
Imitation Spell (3/day) (Reaction). As a reaction, if it is a target of a spell which is on the wizard list, the lich can immediately take the spell's energies and cast it without expending one of its slots.
Improved Defense (Feature). The lich has damage resistance from bludgeoning, piercing and slashing from magical weapons.
Improved Spellcasting (Feature). The lich has from 1st to 3rd level slots double than normal and knows 6 cantrips.
Meta-polymorph (3/day) (Bonus Action). The lich can use a bonus action to assume the shape of an undead. In all other aspects it is otherwise identical to the druid's wild shape.
Painwrack (Action). The lich fixes its gaze on one creature it can see within 10 feet of it. The creature must make a DC 18 Wisdom saving throw, taking 21 (6d6) psychic damage on a failed save, or half as much damage on a successful one.
Regeneration (Feature). The lich regains 10 hit points at the start of its turn. If it starts its turn with 0 hp it doesn't regenerate.
Skull Scry (Feature). The lich can hear or see as the clairvoyance spell through any skull up to 200 miles away.
Voice of Maleficence (Feature). After 1 minute of continuous speech of the lich, a creature that can hear it must succeed a DC 18 Wisdom saving throw or be charmed for 24 hours. While charmed the creature reveals to the lich any secret known. The lich can dismiss the effect whenever it wants.
Vortex of Evil (1/week) (Action). The lich summons fiends that appear in 1d4 rounds in unoccupied spaces that it can see within 120 feet. It can choose
- 1 fiend of CR 15 or lower
- 2 fiends of CR 10 or lower
- 4 fiends of CR 5 or lower
- 8 fiends of CR 3 or lowerThey obey to the lich's telepathic commands, acting in its turn, and they disappear when they drop to 0 hp or are dismissed by the lich as a bonus action.