Thanks Abe
Deineltan, In this case you are incorrect- the phrasing in
myr superion specifies that only mana SPENT on it must come from creatures, without actually specifying that mana MUST be spent at all. This is reasonable because it has a cmc of 2, after all. However,
heartless summoning brings its effective casting cost to zero. You're not spending any mana to cast it and thus where the non-existent mana comes from is a moot point.
Myr superion is a staple in all
heartless summoning decks, I've found, but you get used to needing to explain it to folks who have not encountered it before