Newsoul Amalgam

Newsoul Amalgam enters the battlefield with
a +1/+1 counter on it.
At the beginning of each opponent's turn, they
may pay X, where X is the number of +1/+1
counters on Newsoul Amalgam. If they don't,
double the number of +1/+1 counters on it.
A Simic experiment at growing new souls from scratch
that went a little too well. Fortunately, Syndicate guildmages can control it, for a price.