In keeping with the rest of this week, I’m already posting a revision on my OSR post from the other day. I realized that the version I posted before is probably overpowered by the standards of most old-school games, so I boiled it down to its essence and tried to adopt some Flailsnails-style compatibility.
In whatever retroclone or actual old RPG you are using, remove spellcasting from the cleric class and also remove the weapon restrictions. In addition to Turn Undead, they gain two new abilities. First, they have an X in 6 chance to Detect Chaos (or Evil) within a 100′ radius, where X is equal to their half their level, rounded up. Second, they gain a damage bonus versus undead, demons, and other supernaturally evil creatures, also equal to half their level rounded up.
I think that hits the right point in the power curve.