Add support for quest 6921 in Blackfathom Deeps

* Added script support to make NPC 12876 (Baron Aquanis) spawn (and only
spawn once) when players on the quest 6921 uses GO 177964 (Fathom Stone)
Source: http://www.wowwiki.com/Fathom_Stone
http://www.wowhead.com/quest=6921/amongst-the-ruins