I have always liked the idea of level drain as a representation of terror/horror as the given monster actually scares the PC. I never liked the fact you could be de-leveled back to level 1 – though I never saw this happening back in 1e days, PCs were just too careful when dealing with level-draining monsters. For my 5e game my wraiths, spectres and vampires all energy drain. Here is the specific rule I use.
Negative Energy Drain. First hit drains a level on a failed DC XX CON saving throw – the target doesn’t actually lose a level but does not level up the next time s/he can. Instead, their total is reset back to the current levels xp total. A level 1 character is killed. Each hit thereafter drains 1d4 CON, if the target is reduced to 0 CON they can choose to lose another level or die.
All clerics should know that these monsters level-drain – no skill check required. This way your party is not surprised by this. I also added the following 9th level spell to all good and anti-undead domains:
Negative Plane Protection
Level: 9 (Abjuration, Ritual)
Range: Touch
Duration: 8 hour
Range/Area: One Prime Material Plane creature
Components: V; S, M (100 gp in ritual material)
Casting Time: 10 minutes
Saving Throw: None
This spell enables the caster or any other eligible creature touched to be partially protected from an undead monster that has an existence on the Negative Material Plane (such as a shadow, wight, wraith, spectre, or vampire). The dweomer of the spell opens a channel to the Positive Material Plane, the energy from which helps to offset the effect of the undead creature’s attack. The recipient is allowed a WIS saving throw with a DC equal to the damage dealt if he or she is touched (attacked) by an undead creature. Success indicates that the recipient takes normal hit-point damage from the attack, but does not suffer the drain of experience that would otherwise take place. In addition, the undead creature takes 2d6 hit points of radiant damage from the Positive Plane energy. The magic is only proof against one such attack and dissipates after that attack whether or not the saving throw is successful. If the saving throw is failed, the recipient of the spell takes double the usual physical damage in addition to the loss of experience that normally occurs. The contact between the Positive and Negative Planes that this spell brings about will cause a bright flash of light and a sound like that of a thunderclap, but these phenomena do not cause damage in any event. The spell will cause the cleric to explode (instant and irrevocable death) if he or she dares to cast it while on the Negative Material Plane. The spell will not function at all on other planes of existence besides the Prime Material.