Ancient Earth Elemental

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.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

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

Multiattack. The elemental can make two Fist attacks or Earthbreaker then one fist attack. This increases to three when using Earthen Fury.

Fist. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 26 (3d12 + 7) bludgeoning damage.

Rock. Ranged Weapon Attack: +11 to hit, range 120/480 ft., one target. Hit: 34 (5d10 + 7) bludgeoning damage. If the target is a creature, it must succeed on a DC 18 Strength saving throw or be knocked prone.

Earthbreaker.  The elemental slams the ground with one of it's giant fists causing stone spikes to erupt from the ground within a 30ft. radius. Any creature on the ground within the radius must make a DC 18 Dexterity saving throw. On a failure a creature is knocked prone and takes 2d8 (9) bludgeoning damage. On a successful save they take half damage and are not knocked prone.

Earthen Fury. When the elemental's health hits 50% (136) it activates its Earthen Fury. It sheds some of it's natural armor allowing it to attack with less hindrance. The elemental's AC decreases to 13 but it may make one additional fist attack as part of its multiattack.

The Elemental can take two 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 Elemental regains spent legendary actions at the start of its turn.

Fist.  The Elemental makes one Fist attack.

Earthbreaker.  The Elemental use's its Earthbreaker ability once.

Earthen Spike.  The elemental causes a stone spike—5 feet tall and 1 inch in diameter—to burst from the ground at a point it can see within 30 feet of it.  Any creature in the space where the spike emerges must make a DC 18 Dexterity saving throw. On a failed save, the creature takes 18 (4d8) piercing damage and is restrained by being impaled on the spike. A creature can use an action to remove itself (or a creature it can reach) from the spike, ending the restrained condition.