Making a cycle of Herculean tasks based on Theros myth. This one is from the origin story of Athreos: he was the first mortal to die, and brought each of the monocolored gods an offering. They set him in charge of sorting the souls that would follow him and cast his offerings back into the world as five coins, promising that he could retire once the coins were returned

So you sacrifice five artifacts (mandatory, not optional) to return a single creature or planeswalker? And that's costing you rather than (see Obzedat's Aid)? That's a hard pass.

If you added some kind of minor reward for each artifact sacrificed (e. g. draining 1-2 life from an opponent), then the incremental gain would balance out better.

I'd also make both parts of the task optional i. e. "you may sacrifice an artifact creature" for the first triggered ability and turning the second part into an activated ability.

There is some flavor disconnect, but that's an ambitious story to tell.

That's true. In my mind, you run this in an artifact token deck like treasures, which is a flavor home run (since you're paying Athreos coins)

This cycle was supposed to be based on the Zendikar quests, so I should probably copy their template too