Mordenkainen's faithful phantom guardian

 Mordenkainen's faithful phantom guardian was an uncommon spell that summoned a spectral guardian.

Effects
With this spell, the caster was able to summon a phantom guardian for protection. The guardian watched over the caster from the Ethereal Plane. The guardian was able to see everything around the caster in a radius. It was able to see astral, duo-dimensional, ethereal, invisible, or out-of-phase creatures. If the guardians detected a threat to the caster, they instantly appeared on the Prime Material Plane next to the caster. The guardian did not speak and appeared as a transparent figure outlined with a faint shimmering purple light. It fought to defend the caster until either the threat was eliminated or the guardian itself was destroyed. The caster was able to summon the guardian to their side at any time after the spell was cast. This spell was unable to be cast in the phlogiston.

Components
In addition to verbal and somatic components, the spell required a material component: an expensive diamond, which had to be valued at at least 1,000 gp and was given to the guardian during casting.