Spawn Gunther's Visage (npc 5666) at end of quest 411

Script to run after completing quest 411 The Prodigal Lich Returns. C++ code by @kldxzzhc2 in  #12482
Modification: Added actual NPC text from http://www.wowwiki.com/Gunther_Arcanus .
don't use pointers as class member fields, they might become invalid