When Syndicate Messenger dies, its afterlife ability and Nim Deathmantle trigger.
Resolve the Syndicate Messenger trigger, creating a 1/1 Spirit creature token.
Activate Ashnod's Altar by sacrificing the Spirit, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Syndicate Messenger from your graveyard to the battlefield, and attach Nim Deathmantle to it.