Anyways it appears I was right about dungeon being outside the deck.
I'm not entirely sure how this interacts with Commander and Brawl, once ventured into the Dungeon goes into a Command Zone, but till then its in the side board (which Commander & Brawl don't have) but doesn't use a slot. I suspect it will work simularly to Companions in Brawl and Commander.
Also this plays to my belief that Mythals are the new enchantment subtype.
And I plan on making an Ellywick Brawl Deck for Arena.