Omnath is a wild magical elemental representing Zendikar's abundant mana. She is a destructive being composed of chaos itself, featuring into many Zendikar creation myths, and is believed to be the origin of all mana on the plane- in effect being the culmination of all the plane's leylines given form. Omnath was originally sealed in a prison that needed to be tented to every twoish years, but the Rise of the Eldrazi broke her free and now she's running wild, slowly absorbing mana into herself until she becomes stronger and stronger, causing chaos in her wake as she does her best to fight against the Eldrazi threat. She's finally gotten a bit of black mana from Phyrexia compleating her, but with New Phyrexia phased out he's likely out cold.
I don't have the ability to comment on deck building.