Here's an older artifact, fairly generic. Change around the open-ended spell casting for specific, celestial/elemental/narratively fun spells based on whatever character level you want. Essentially, it becomes something too important to be parted with, as it makes any being that bears it a high level caster. 3.5e had weird mechanics around lost levels, but I think 4 lost levels just meant a -4 on all rolls or roll with disadvantage. It's in no way balanced, but it does have stats.
The Shield of the Sun: This +3 large shield, emblazoned with the symbol of the sun, allows the wielder to cast spells as if she were a 20th-level paladin with a Wisdom score of 20. The spells gained are cumulative with any existing spells per day that the character might have, even if she’s already a paladin. The Shield of the Sun also grants advantage against spells to its wielder. It absorbs the first 10 points of damage from any energy attack (fire, cold, acid, electricity, or sonic). In return for all this, once per year the shield’s owner must undertake a quest (no saving throw to avoid) at the behest of a lawful good deity.
A character who is evil or chaotic (LE, NE, CE, CN, CG) gains four negative levels if she attempts to use this artifact. Although these negative levels never results in actual level loss, they remain as long as the shield is in hand and cannot be overcome in any way (including restoration spells). The negative levels disappear when the shield is stowed or leaves the wearer’s possession.