Legendary Resistance (2/Day). If the Shield Master fails a saving throw, it can choose to succeed instead.

Adamantine Coating. Any critical hit against the Shield Master becomes a normal hit.

Obstacle. A medium or smaller creature has three quaters cover against attacks or effects that orinate from the other side of the Shield Master

Regeneration. The Shield Master regains 10 hit points at the start of its turn if it has at least 1 hit point.

Spell eating amulet. The Shield Master wears an amulet that grants him acces to cast Counterspell 2/Day

Multiattack. The Shield Master makes two attaks with his Shield smash.

Shield Smash. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 18 (3d6 + 8) bludgeoning damage. The target must succeed on a dc 18 strength saving throw or be knocked prone. 

Stand behind me. When an ally of the Shield Master would be hit by a ranged attack within 15ft. of it. He can jump between the ally and the attack. The attack trys to hit the Shield Master instead.

The Shield Master 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 Shield Master regains spent legendary actions at the start of its turn.

Deflect (Costs 1 Actions). The Shield Master angles its shield, increasing its AC by 2 for the duration. Any ranged attack that misses the Shield Master instead hits a creature within 15ft. of it. This effect lasts until the Shield Masters next turn or until it uses a other legendary action.

Ram (Costs 1 Actions). The Shield Master moves up to its speed in a single direcion, attempting to pass through any number of Larger or smaller creatures spaces. Each creature it tries to pass through must make a DC20 Strength or DC 17 Dexterity saving throw. On a failed save the creature is knocked prone and takes 3d8 bludgeoning damage. On a succesfull strength saving throw the creature takes half as much damage is knocked prone, and reduces the shield guardians speed to 0 until the start of its next turn. On a succesfull dexterity saving throw the crature takes no damage an isnt knocked prone. The Shield Master cant move through Huge or larger creatures spaces.

Sweep (Costs 2 Actions). The Shield Master swings its shield in a whirlwind of adamantine. Each creature within 10ft. of it must make a DC18 Dexterity saving throw. On a failed save the creature takes 6d8 bludgeoning damage and is knocked prone. On a succesfull one the creature takes half as much damage and isnt knocked prone.