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 .
Use CAST_AI() only when you want to check if the AI is of the expected type (comparing the result to NULL), otherwise use ENSURE_AI() when you are 100% sure it will always be of the expected type.