Thornbite Staff attached to Priest of Forgotten Gods.
Priest of Forgotten Gods does not have summoning sickness.
You control at least one additional creature.
Steps
Activate Priest of Forgotten Gods by tapping it, sacrificing Reassembling Skeleton and any other creature.
When Reassembling Skeleton and the other creature die, Priest of Forgotten Gods triggers twice, untapping Priest of Forgotten Gods.
Resolve the Priest of Forgotten Gods ability from step 1, causing any number of players to lose 2 life and sacrifice a creature, and you to add ({B} magic symbol) ({B} magic symbol) and draw a card.
Activate Reassembling Skeleton by paying ({1} magic symbol) ({B} magic symbol) , returning it from your graveyard to the battlefield.
Insidious Roots triggers, creating a 0/1 Plant creature token, and then putting a +1/+1 counter on each Plant you control.
Repeat.
Results
Infinite card draw.
Infinite draw triggers.
Near-infinite +1/+1 counters on Plants you control.