The base I used for the Ur-Dragon was the statblock provided for Tiamat in the Rise of Tiamat module. To mirror its abilities both on the card and in the lore, I made a lot of the Ur-Dragon’s abilities ignore whether its target is on another plane. The Call Brood ability is one I’m particularly proud of; it allows the Ur-Dragon to summon either a group of smaller dragons, or something more powerful, up to an adult red dragon. The Eternal Sight, Eternity Spanning and Wing Attack abilities all further this theme, giving the Ur-Dragon easy interaction with other planes, and a way to sweep its foes away through the aether of the multiverse.
To see other creatures of Commander 2017, click here for my masterpost!