Activate Phyrexian Altar by sacrificing Workhorse, adding one mana of any color.
Workhorse dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return Workhorse from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Workhorse enters the battlefield with four +1/+1 counters on it.
Warstorm Surge triggers, dealing 4 damages to any target.