Homebrew
Aversion to Hostility. The guardian initially does not attack creatures unless they pose a direct threat to the Nexus Heart. If the guardian is attacked or witnesses an attack on the Nexus Heart, it enters combat mode. Immutable Form. The guardian is immune to any spell or effect that would alter its form.
Multiattack. The guardian makes two melee attacks. It can forgoe one of the melee attacks to instead use its Arcane Blast Slam. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 5) bludgeoning damage. Arcane Blast. The guardian releases a burst of arcane energy. Each creature in a 15-foot cone originating from the guardian must make a DC 14 Dexterity saving throw, taking 24 (7d6) force damage on a failed save, or half as much damage on a successful one. Defensive Pulse (Recharge 5-6). The guardian emits a pulse of energy in a 30-foot radius. All creatures within the area takes 12 (4d6) force damage and must make a DC 14 Strength saving throw. On a failure, the creature is knocked back 20 ft. from the guardian and are knocked prone Arcane Resonance. The guardian can manipulate arcane energies within a 30-foot radius centered on itself. As a bonus action, it creates an area of arcane resonance. This zone grants all creatures within advantage on Intelligence (Arcana) checks and allows them to cast spells requiring verbal components without speaking. Resonance Link. The guardian can establish a magical link between creatures within 30 feet of itself. As an action, it can choose two creatures within range to create a telepathic connection. These creatures can communicate with each other telepathically as long as they remain within 1 mile of the guardian. The link lasts for 24 hour(s). Nexus Mend. The guardian can channel the Nexus's restorative energies to mend objects. As an action, it can touch a non-magical object no larger than 10 cubic feet. The object is repaired, regaining 10 hit points. If the object used to be magical, the guardian can call upon the Nexus to restore its magical properties, this takes 12 hours of uninterrupted stasis.
The Ancient Nexus Guardians reside within the Nexus Heart chamber, guarding it from any potential threats.