King Arthur

While King Arthur rules the realm, if your opponent would control a card you own, you control it instead (applying equally for both players). For example:

• My opponent plays Lilith then summons a minion from my deck. My opponent would normally control a minion they summoned, but I control it while King Arthur is in the realm.
• I play Heirloom Lost, Postern Gate, Desecrate to get a Desecrated Ground, etc. My opponent would normally control that site I own, but I control the site while King Arthur is in the realm.
• I play any Relic (e.g. Ring of Morrigan) and my opponent takes it somehow (e.g. Swap). My opponent would normally control that relic while carrying it, but I control it while King Arthur is in the realm.
• Even if I control it, my opponent's unit is still the bearer and can benefit from bearer abilities.
• Since I control it, terms like "ally" and "enemy" apply from my perspective (the controller).
• If it's dropped, no one controls it, as normal.
• I animate my aura via Enchantress. You play Mesmerism on it. Even after it returns to being a normal aura, you would normally control it, but I control it while King Arthur is in the realm.

Cards that have different element symbols count as a spell of each of those affinities. So Arthur and Guinevere are each an Air spell, an Earth spell, a Fire spell, and a Water spell, all at the same time. But you are still casting them together as a single spell. So, assuming Arthur is your spellcaster’s first spell of the turn, the Stone will offer a discount of 1 mana for Arthur, making him cost 3 mana. This also means that when Arthur is played he uses up the Stone for that spellcaster's turn because he’s simultaneously the first Air, Earth, Fire, and Water spell played this turn. So the Spellcaster cannot continue to use the stone that turn.