Looking on the wiki, it seems to indicate that with the Sun Domain undead will be damaged, rather than feared, with a Turn Undead. My cleric has Healing and Sun domains and has never dealt damage with Turn Undead. I'm not sure if the ability just isn't working correctly or maybe it's because I chose Healing and Sun as my domains instead of Sun first, Healing second.
Sun Domain
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Re: Sun Domain
The Arelith wiki says that turn undead "deals Positive and Fire damage to target if it cannot be Commanded". Have you been trying to command undead?
Turn undead has unique functionality on Arelith and has its own wik page explaining that.
https://wiki.nwnarelith.com/Turn_Undead
In brief, you can specify the effect you want: Command, Damage, Destroy, Fear, Neutralize, or Rebuke.
Re: Sun Domain
I've been using Turn Undead as a Neutral Good healing/sun domain cleric with improved turning and roughly 30 cha now at level 29. I've never dealt damage. If there's some other method of turning that involves commanding undead I've not seen it.