Baneguard is an undead monster which originally appeared in the Monstrous Compendium Annual - Volume I (AD&D 2nd edition rules) published in 1994 by TSR.
You can find there a conversion to D&D 5th edition, ready to use.
Baneguards are from the Forgotten Realms setting, created by a spell of the Church of Bane, but during the years the secret of this spell has been spread through the Multiverse that now many evil deities allow their clerics to use it.
Therefore, you can easily introduce this monster in any campaign of yours.
The Baneguard is a skeleton, usually but not always human, which is animated by a spell to serve as a guardian.
Guardian. The Baneguard is used by evil clerics or churches to guard places like holy sites or hidden temples.
Intelligent Servant. The Baneguard is much more intelligent than common skeletons and it can act independently from its master, within the limits of the orders it has received.
Undead Nature. The Baneguard doesn't require air, food, drink or sleep.
Medium or Small undead, neutral evil
Armor Class 13 (natural armor)
Hit Points 39 (6d8 + 12)
Speed 30 ft.
STR 10 (+0) DEX 14 (+2) CON 15 (+2) INT 12 (+1) WIS 12 (+1) CHA 11 (+0)
Skills Perception +3 Religion +3
Damage Vulnerabilities bludgeoning
Damage Immunities poison
Condition Immunities exhaustion, poisoned
Senses darkvision 60 ft., passive Perception 13
Languages understands any languages spoken by its masters but can't speak
Challenge 1 (200 xp)
Innate Spellcasting. The Baneguard's innate spellcasting ability is Intelligence (spell save DC 11). It can innately cast the following spell, requiring no material components:
Shortsword. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 5 (1d6 + 2) piercing damage.
Magic Missiles (Recharge 5-6). The Baneguard creates four glowing darts of magical force from its bony fingertips (or any extremity if fingers are missing). Each dart hits a creature it can see within 60 feet. A dart deals 1d4+1 force damage to its target. The darts all strike simultaneously, and it can direct them to hit one creature or several. Shield or Counterspell (cast with the lowest level slot needed) negate the effects of the darts.