Zur the Puppeteer

Zur the Puppeteer enters the battlefield with {x} +1/+1 counters.
Spells you cast with {x} in the mana cost now cost {x} less to cast where {x} is the number of +1/+1 counters on Zur.
If a spell with {x} in its mana cost would be cast
by you without spending mana, {x} = 10.