Actions

Multiattack. The sentinel makes three melee attacks or two ranged attacks.   Force Slam: Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 18 ( 2d10+5 ) force damage.   Arcane Bolt. Ranged Spell Attack: +8 to hit, range 24 sq., one target. Hit: 20 ( 4d6+6 ) force damage.   Binding Chains (Recharge 1d6>=5 ): The automaton releases chains of magical energy. Each creature in a 4-square radius must make a DC 16 Dexterity saving throw or be restrained for 1 minute. A restrained creature can make a DC 16 Strength saving throw at the end of each of its turns, ending the effect on itself on a success.

Bonus Actions

Sentinel's Mark. The sentinal can mark any creature that took a document from the library with the Sentinel's Mark while it knows its location. While the mark is active the sentinel can   Dash. The sentinel moves up to its movement.

Reactions

Shield (1st Level Slot). An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.   Counterspell (3rd Level Slot). You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.