"You see this."

The basic game does not specify a lot of aspects of the Cleric’s ability to turn undead. My opinion on this is simple: Each edition of the game is its rule set, so when the rules don’t limit an ability, I consider the absence to be deliberate.


To put this in perspective, consider something we take for granted, like Infravision. Elves and Dwarves are able to see up to 60′ with it. The rules don’t specify a limit to the number of times per day they can use their infravision, how long they can use it, or how far they can move while using it. And if your GM were to suddenly propose such limitations on your PC, consider why you might object.

As a reminder, this is all we get, description-wise, in OSE and BX:
OSE: Clerics can invoke the power of their deity to repel undead monsters encountered.
Turned undead: Flee from the cleric’s presence if possible, and will not harm the cleric.
BX: If a cleric Turns an undead monster, the monster will not touch the cleric and will flee from the area if it can.

1. The frequency with which a Cleric can use their Turn ability is not specified. Meaning, there is no limitation placed on the number of times per week, per day, or per Turn. Per Round is inferred, because characters can not take more than one action per round unless magically aided.
   
   
2. The duration of a successful Turn effect is not specified. (Therefore, I consider it a permanent effect.)
   
   
3. The range (distance or AoE) of the Turn effect is not specified. (Therefore, I consider it line of sight.)
   
   
4. The conditions of the Turn effect are not specified: Whether it ends if a monster is attacked, or touched, or runs out of room to flee. Whether the undead can make attacks on other PCs while moving away from the Cleric. (Therefore, I consider it conditional: If a PC makes themselves an obstacle to a fleeing undead, it will attack while continuing to try to move away from the Cleric. No more, no less.)
   
   
5. Whether a failed Turn can be re-attempted on the same monster is not specified. This is huge and quite deliberate: There are rules in BX for other skills and abilities that eliminate ‘roll until you make it’ mechanics, but not here. (Therefore, I consider it repeatable. A failed attempt is already a penalty paid; the Fighter is allowed to miss and attack again, so too is the Cleric.)
   
   
6. The phase of combat in which Turning occurs is not specified. One could narrate and rationalize it to fit any phase, because it can arguably be considered a ranged attack (because it certainly happens at a distance), a magic attack (because it is a divine power), a melee attack (if one imagines wielding a holy symbol) or even a non-phased conversational action that happens before initiative (allowing it to presuppose morale rolls) or as a Slow action (last, as though losing initiative, if one imagines the act to take time and dialog). Of these options, I choose to consider it a magical attack.
   
   
7. The actions of the cleric are not specified in any way. Must they move to front of the party? Are they even permitted to move, or is it like casting a spell? Must they speak, or is willpower enough? Must they hold their holy symbol in a hand, or can it be worn? Do they need line of sight on the monsters? If so, does that imply they need light to see the undead, or is it still possible in the dark? Literally none of this is written. (Therefore, I make no new limitations: The cleric can Turn by willpower so long as they have their holy symbol upon their person. Period.)
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8. Only the cleric is mentioned by name as being regarded fearfully by the undead. How the undead regard the rest of the party is not specified. (Therefore, as above, I consider them to have no regard for the party unless blocked from fleeing, because that is their new primary objective.)
   
   
9. If lesser undead in a mixed group are automatically Turned (T) or Destroyed (D), do they still absorb hit die from the 2nd roll? This is not specified, but “No” is deductible: If we say “Yes, it just automatically succeeds the first roll, but the second still determines the number affected,” then we realize this logic fails: 6 zombies are 12HD, and if zombies should automatically be turned or destroyed, then a 2nd roll of 8 would reverse the automatic effect on two of the zombies. Therefore, in a mixed group (say, a bunch of zombies blocking for two Mummies), then all the zombies are automatically affected, and the first roll (for success) only applies to the mummies, and the second roll (for HD affected) really only determines whether the 2nd mummy flees, because on a successful first roll, at least 1 monster is affected regardless.
   
   That’s quite a bit to unpack if you’re a novice to the edition, but unpack we must.
   
   
10. When undead are Turned, their trajectory is not specified, but “fleeing from the cleric’s presence if possible” seems to indicate that they will proactively find a point of egress, even if that exit is behind or near the cleric. This is not the same language as “move away from the cleric” which could get tricky when it comes to corners and angles. (Therefore I understand “fleeing from the cleric’s presence” to mean “out of the area, as far and as fast as possible.)
    
    

Also, good reminders:

1. Initiative and movement rates still apply to turned undead. So when a monster flees, it does so on its movement phase per initiative, not instantly when the Cleric Turns them.
2. Turned undead are not “retreating” unless they were engaged in melee from which they are retreating. In which case, they are then subject to +2 to be hit by their melee opponent, as they turn their back to retreat.
3. If the path away from the cleric is blocked by a PC in melee, fighting withdrawal movement rates may apply; the undead needs to fight their way around another PC while backpedaling.