Bloodspore Thrinax has at least five +1/+1 counters on it.
Steps
At the beginning of your end step, Soulherder's second ability triggers, blinking Sage of Hours and causing it to enter the battlefield with five or more +1/+1 counters on it due to Bloodspore Thrinax.
Soulherder's first ability triggers, putting a +1/+1 counter on itself.
Activate Sage of Hours by removing all +1/+1 counters from it, causing you to take at least one extra turn after this one.