You make quest you enter beleth and you go to Beleth's strong room. You find the real beleth and after you kill it, Beleth Presentation Elf appears (ID:29128). Normally you talk to this Npc and it gives you permission to Enter the Stone Coffin (ID:32470), where you talk again and you take beleth ring.
In L2j Files, after you talk to Beleth Presentation Elf appear the dialogue that should appear when you speak to Stone Coffin. Beleth presentation Elf gives you beleth ring but is not right. Beleth Presentation Elf just gives you the permission to enter coffin and take the drops. take a look at this