The main new ideas here are 1) clerics take damage on turn attempts that fail and 2) holy symbols serve as spiritual armor to reduce that backlash damage. Holy symbols also become damaged on bad turn rolls just like I have been having mundane gear degrade on bad combat rolls. The consequences of a failed turn roll are intended to provide a cost to using turn undead continuously without resorting to spell slots or some separate resource (as Pathfinder does with “channel energy” rules).
I have this vision of a cleric’s silver holy symbol melting in the face of a horde of wraiths.
The text below is an excerpt from a longer house rules document that I put together for an in-person game, and as such some terms are referenced without being defined. Overkill, for example, means combat roll success margin of 4 or better (or a natural 20).
By brandishing a holy symbol, clerics may attempt to overawe the undead. Turning undead affects a number of monster levels equal to 1d6 plus cleric level (the player may select which monsters are affected). It is resolved by a combat roll using cleric level rather than combat rating. Overkill results on a turn roll additionally deal 1d6 damage.
Undead that are affected by a turn shrink back in terror, and keep well away from the cleric as long as the turn is maintained. Such overawed creatures may not attack by any means. A turn may be maintained as long as the cleric does not engage in any activities other than concentration and slow movement. Attacking turned undead frees them from the effect.
Attempting to turn undead pits a cleric’s soul against the undead’s power. If the turn fails, the cleric takes 1d6 damage. This damage is spiritual, and so is not reduced by armor. Clerics reduced to zero HP from such spiritual damage are not slain, but rather knocked unconscious, retaining a single hit point.
Holy symbols become damaged in the case of low combat rolls just like weapons. Particularly fine holy symbols also serve as spiritual armor. Silver provides DR 1, gold DR 2, and platinum DR 3. This DR only applies to damage from failed turn attempts.
Characters of any class may assist with turning undead by brandishing a holy symbol. An assistant adds 1d6 to the effect coverage but no bonus to the combat roll. Assistants also take damage from failed turn attempts and must similarly concentrate to maintain a turn effect.