Alpheios

Crystalline Bulwark. At the start of the genasi's turn, it gains 100 temporary hit points. While those hit points remain, the genasi has a plus 5 to its AC.

Crystalline Terrain. Difficult terrain created by Alpheios deals 5 (2d8) piercing damage for every 5 feet a creature moves through it. 

Earth Glide. The genasi can burrow through nonmagical, unworked earth and stone. While doing so, the elemental doesn't disturb the material it moves through. It can also move through difficult terrain without expending extra movement. 

Legendary Resistance (3/day). When the genasi fails a saving throw, it can instead succeed. 

Vanguard's Defense. The genasi reduces the damage it takes from each attack by 6.

Reactionary. Alpheios has 4 reactions every round. 

Spellcasting. The genasi casts spells using Wisdom (spell save DC 18, +10 to hit with spell attacks), and knows the following spells prepared:

At will: earth tremor, mold earth.
3/day: erupting earth, shatter, each at 5th level.
1/day: wall of stone.

Multiattack. Alpheios makes four attacks, and it can replace two of these attacks with a spell.

Tectonic Punch. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 26 (3d10 + 9) bludgeoning damage and must succeed a DC 18 strength saving throw or be pushed 15 feet. 

Supersonic Pebble [2/turn]. Ranged Weapon Attack: +15 to hit, range 30/60 ft., one target. Hit: 23 (4d6 + 9) bludgeoning damage and must make a DC 18 constitution saving throw or be stunned. 

Get Out of Here. When a creature misses the genasi with an attack roll by 5 or more, the creature must make a DC 18 Strength saving throw or be thrown 15 feet. 

Tectonic Rush. The gensasi dashes 60 feet, leaving difficult terrain in a 10 foot wide line behind it.

Get Over Here. The genasi casts earthbind. 

Tend the Garden (2 actions). The genasi creates an area of area of difficult terrain with a 30 foot radius. A creature that enters this area for the first time this turn must succeed on a DC 18 Strength saving throw or be restrained until the end of its next turn and takes takes 28 (8d6) piercing. On a success the are not restrained and regain 14 (4d6) hit points.