When Pelakka Wurm dies, its second ability and Nim Deathmantle trigger.
Resolve the Pelakka Wurm trigger, causing you to draw a card.
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Pelakka Wurm from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Pelakka Wurm enters the battlefield, its first ability triggers, causing you to gain 7 life.
Trudge Garden triggers, causing you to pay ({2} magic symbol) to create two 4/4 Fungus Beast creature tokens.