Bobby

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

Magic Resistance. The bobby has advantage on saving throws against spells and other magical effects.

Dungeon Knowledge. As long as the bobby's body is touching the floor, ceiling, or walls of the dungeon, it knows the location of every creature inside the dungeon that it's guarding. In order for the creature to be located in such a way, it also requires it to be in contact with walls, ceiling or floor of the dungeon, levitation or flight are effective ways to stay undetected.

Identify Intruders. The bobby seeks and attacks creatures that break the rules of the dungeon, or that harm it in some way. Examples of this are littering, vandalism, taking things outside of the dungeon. At the DM's discretion, more actions can be set as rules. Once a creature has been identified as a lawbreaker, the bobby will hunt it tirelessly, in addition any creature seeking to stop the bobby from punishing a lawbreaker will also be identified as such.

Earth Glide. The bobby can burrow through nonmagical, unworked earth and stone present in the dungeon. While doing so, the elemental doesn't disturb the material it moves through, this trait can be used exclusively inside the dungeon it guards.

Trap Flooring. Every 5 foot square the bobby walks on becomes a trap, stepping on it triggers a DC 15 Dexterity save, on a failed save, the flooring snares the target, the target is then restrained. A creature restrained by the trap can use its action to make a DC 20 Strength check, if it succeeds it is no longer restrained, alternatively a creature can break free by destroying the flooring. It has AC 15 and 25 hit points, and it is immune to poison and psychic damage.

Adamantine construct. The bobby is immune to critical hits.

Multiattack. The bobby makes two cudgel attacks, or one cudgel attack and one go to jail attack.

Cudgel. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 18 (3d8 + 6) bludgeoning damage, on a hit the creature has to make a DC 15 Constitution save, on a failed save the creature is stunned.

Go to jail. The bobby attempts to grapple the creature it just attacked with it's cudgel attack, on a success the bobby can teleport the target to a locked cell, and itself to the prison room inside the dungeon as a bonus action. It will always verbally state before doing so that the grappled creature is going to jail.

Determine trajectory. When the bobby is attacked by a ranged attack or a ranged spell attack, it can use its reaction to make a Wisdom (Insight) check, if it's check is equal or beats the hit roll from the ranged attack or ranged spell attack, the bobby dodges it.
This trait doesn't allow the bobby to dodge the area of effect of a spell, it doesn't work if the spell or ranged attack hits automatically, it also doesn't work if the spell or ranged attack takes up the entire space of the bobby's square.