Six mana for a 5/5 was a decent rate for the era, and protection from white was extremely relevant
The posture of the two characters does a lot of heavy lifting here, the cold assuredness of the Eternal perfectly balanced by the complete vulnerability of the Aven
Aaron called these Commands and made a rare cycle of them
Getting to put X creatures of the same type on the battlefield is huge for a 7-mana investment, but at the same time, you need to have a board, otherwise youre not getting enough from this green instant
A three-mana combat trick is a big ask, but this will make a huge difference in a race, and because it cantrips, is going to slot nicely into the UB draw-two archetype.
The cards are too expensive for Wizards to put them in a Commander precon, but they'll likely want to increase supply before Lost Caverns of Ixalan releases, which makes Commander Masters a pretty obviously landing spot