Boulder-Geist

Earth Glide. The elemental can burrow through nonmagical, unworked earth and stone. While doing so, the elemental doesn't disturb the material it moves through.

Siege Monster. The elemental deals double damage to objects and structures.

Ethereal Sight. The ghost can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.

Legendary Resistance (3/Day). If the Boulder-Geist fails a saving throw, it can choose to succeed instead.

Multiattack. The Boulder-Geist can use two weapon attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 14 (3d8 + 5) bludgeoning damage.

Withering Touch. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 17 (3d6 + 3) necrotic damage.

Hurl Rock. Ranged Weapon Attack: +7 to hit, range 60/240 ft., one target. Hit: 28 (3d10 + 6) bludgeoning damage.

Consume Ghosts. The Boulder-Geist consumes all ghosts summoned by it, causing them to disappear, and the Boulder-Geist gains 2d4 hp per ghost consumed

Horrifying Visage. Each non-undead creature within 60 feet of the Boulder-Geist that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target also ages 1d4 × 10 years. A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to this ghost's Horrifying Visage for the next 24 hours. The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

The Boulder-Geist can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Boulder-Geist regains spent legendary actions at the start of their turn.

Earthquake. The Boulder-Geist makes an earthquake. Each creature within 30 ft. of the Boulder-Geist, and on the ground, must make a DC 17 Dexterity saving throw or take 21 (4d8 + 4) Bludeoning Damage.

Attack. The Boulder-Geist makes one attack with its Slam, or Withering Touch

Summon Ghosts (Costs 3 Actions). The Boulder-Geist summons 1d4 ghosts, which each appear in unoccupied spaces within 60 feet of the Boulder-Geist, act as its allies, and can't summon other ghosts. A ghost disappears when it is reduced to 0 hit points, or when Boulder-Geist uses its Consume Ghosts action. When the Boulder-Geist uses this legendary action, it can't use it again until all ghosts summoned by it have disappeared.