Arcades Sabboth

Arcades Sabboth was a mysterious figure from the Elder Dragon Wars. An Elder Dragon himself, Sabboth was one of the victors of the war, along with his brothers Nicol Bolas, Chromium Rhuell, Palladia-Mors, and his cousin Vaevictis Asmadi. He presumedly died in a battle between the planeswalkers Leshrac (who summoned the dragon) and Kristina, who killed him.

The story comes to us from during the Summit on the Null Moon, where planeswalkers are recalling their conflicts. Ravidels said, that one century earlier Lesrac had summoned brother of his friend Chromium to his side, during his duel with Kristina. Kristina admitted she killed the Elder Dragon in that duel. Chromium became angry at Kristina when she recounted this story, so it's presumed to be true.

It doesn't say clearly which Elder Dragon was summoned, only it was Chromium's brother. So,

• There were six Elder Dragons: Nicol Bolas, Chromium Rhuell, Arcades Sabboth, Palladia-Mors, their cousin Vaevictis Asmadi and Piru.
• Chromium was present on the Null Moon; Palladia was female; Piru was killed in Dihada vs. Dakkon duel; Asmadi was their cousin. They are out.
• We're left with Arcades Sabboth and Nicol Bolas. Since Nicol Bolas is still alive, its presumed to be Arcades Sabboth that was killed.

However, its commonly believed that summoning an unique creature like Arcades Sabboth isn't actually summoning them across the Æther, but instead making a copy of them out of mana. If this is the case then Chromium was only upset with Kristina because she bested one of his brothers, not because she killed him. Additionally, because Leshrac is an planewalker that primarily used black mana, its not entirely clear why he would summon a dragon like Arcades Sabboth, and it may have been a unnamed Lesser Elder who was like a brother to Chromium.