Ancient Sentrybot

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

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.  Magical attacks suffer disadvantage when targeting the golem.

Magic Weapons. The golem's Spear attacks are magical (+1 effective enchantment).

Safety Protocols.  The golem will not attack fleeing creatures, creatures using Force magic/effects, or creatures that have been incapacitated (0 hp or otherwise rendered unable to fight).  If it is attacked by any of these creatures, their "safety" is revoked!

Preservation Protocols.  The golem will seek to retreat to its cubby for repair if reduced to 25% HP.

Fluid Supply.  The golem carries an internal supply of special purpose fluid depending on its "job".  When fully stocked, it has 5 charges of this fluid; it regains one charge every hour.

Combat Fluid - Safety Foam. This sentrybot is supplied with "Safety Foam".  Any form of poison, acid, fire, or lightning caught in the affected areas is snuffed / extinguished / countered automatically if nonmagical; if magical, treat as if affected by a dispel magic of caster level 5.  Any creatures who are affected suffer the following effects:

• Initial failure:  Target is slowed as if by the spell.  The foam will immediately start hardening.  The creature (or an ally) must spend an Action to attempt to remove the foam (DEX saving throw DC 10).  If they fail, move to the next step; if they roll a Nat 1 on the attempt, or do not attempt to remove the foam, move down two steps!
• Second Failure:  The target is immobilized.  Attempting to remove the foam now requires a STR ability check of DC 13, or inflicting 10+ points of slashing damage (the affected creature will take half of this damage).  If the foam is not removed, move to the third step.
• Third Failure:  The target is restrained, and cannot free itself.  The foam can be removed at this stage by some other creature by making a STR ability check DC 16, or by inflicting 20 damage to the foam (it resists slashing and piercing, and is immune to poison and all energy types except Force and Necrotic); the target takes one quarter of the damage inflicted.  After three rounds, move to the next stage.
• Final Stage:  The target is petrified.

Multiattack.  The Golem can make two attacks per round, mixing Spear, Slam, and Project Force as it chooses.

Slam. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage.  (Also, see "Restrain Intruder" bonus action.)

Spear. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 16 (3d6 + 7) piercing damage.

Project Force (Thrown Spear). Ranged Weapon Attack: +11 to hit, range 30/90 ft., one target. Hit: 14 (2d6 + 7) piercing damage.  The spear then dissolves, reforming in the golem's hand at the end of its turn.

Suppress Volatiles (Recharge 5-6).  The golem targets a square within 20' and sprays "safety foam" in a line from its hand to the target square.  Creatures caught in the line, the square, or the squares adjacent to the target square must make a DEX saving throw DC 11 or be affected by the foam (any creatures not in the target square have advantage on this save).  The golem will use this attack on any source of poison, acid, fire, or lightning - including creatures who obviously are creating these things!

Restrain Intruder:  If the golem successfully hits a target with a Slam attack, it can make a Grapple attack against that target as a Bonus Action.

Repulse Intruder:  If the golem successfully hits a target with a Slam attack, it can make a Shove attack against that target as a Bonus Action.

Fluid Wash. The golem can expend 1 charge of its "Safety Foam" fluid to instantly extinguish any ongoing fire, poison, acid, or lightning damage on itself.  The foam has no other effect when used this way.  This option is not available if "Suppress Volatiles" is not ready; using "Fluid Wash" sets "Suppress Volatiles" on recharge.