Puresteel Champion

Vigilance, Indestructible
Convoke (Your creatures can help cast this spell.
Each creature you tap while casting this spell pays for
{1} or one mana of that creature's color.)
When Puresteel Champion enters the battlefield and
an artifact creature helped casting it, champion it.
(A championed card is legendary and gets +1/+1 for
each championed card you control.)