Reinvigorating Spirit (new monster)

Hamlet and the Ghost (1789) engraving by Thew, after Fuseli

"I am thy father's spirit,
Doom'd for a certain term to walk the night,
And for the day confined to fast in fires,
Till the foul crimes done in my days of nature
Are burnt and purged away."

An idea for restoration of drained levels in a low level campaign:

Reinvigorating Spirit

Move: 90 feet/turn

Hit Dice: variable (1-6)
Armor Class: 3
Treasure Type: nil
Alignment: lawful good

Attacks: 1
Damage: 1-6

Undead spirits that committed unjust acts during life and seek atonement in the after life. They appear as wraiths but draw power from the Positive Material Plane. Their crackling, energetic touch restores lost energy levels drained by the hateful undead. No hit points are regained, and the touch actually does 1 die of damage per level restored. Each level restored reduces the spirit's hit dice by one, and when they reach zero hit dice their onus is abated and they are dispelled. They will only restore levels to characters judged worthy, and may require completion of a task beforehand. Some will speak but others communicate only through gestures or are non-communicative. If attacked, they are hit only by magical weapons, and they will respond with their searing touch. They can be turned only by evil clerics.