Crystal Armadillo

Impenetrable Armor: This creature's dense crystalline armor helps protect it from sudden attacks. It has immunity to slashing type damage, and resistance to area of effect magic attacks. However, due to the material the shell is made out of, the creature is vulnerable to Thunder, Force, Bludgeoning damage, or anything a DM could reason would transfer energy through it's armor.  ie: a loud screech

Magic attacks that would target the creature personally rather than having to pass through it's, such as "mind-spike"  ignore this resistance.

Multi-attack. The Crystal Armadillo makes two claw attacks against the same target or one each versus two adjacent targets.

Claw Attack: +6 to hit, reach 5 ft., 1-2 target. Hit: 8 (1d6 + 4) slashing damage.

Headbutt: +5 to hit, reach 10 ft., 1 target. Hit: 9 (2d4 + 4) bludgeoning damage.

Spinning Slam: +4 to hit, reach 5ft, 1 target. Hit: 20 (3d10+4) bludgeoning damage, and .3 (1d6) piercing damage. The target must make a DC 13 STR saving throw, or be knocked prone. *Requires previously using Legendary Action "Spin"* 

Spin: (Costs 1 Actions): This creature can curl up into a defensive ball, and start spinning in the direction it was previously facing. While spinning the creature has resistance to all attacks coming from the opposing direction, it's vulnerabilities now deal normal damage.

Spinning Turn: (Costs 1 Actions): This creature can adjust the angle/direction of it's spin without ending it.

Prolonged Endurance: (Costs 1 Actions): This creature can automatically succeed on a STR, or CON saving throw, or automatically end an effect against it at will.