Activate Skirk Prospector by sacrificing itself, adding {R}.
Skirk Prospector dies, triggering Rundvelt Hordemaster and Mortuary.
Resolve the Mortuary trigger, putting Skirk Prospector on top of your library from your graveyard.
Resolve the Rundvelt Hordemaster trigger, exiling Skirk Prospector and allowing you to cast it until the end of your next turn.
Cast Skirk Prospector from exile by paying {R}.
Repeat.

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Infinite storm count

Rundvelt Hordemaster

Skirk Prospector

Mortuary

Activate Phyrexian Altar by sacrificing a Goblin that can be cast using one colored mana, adding one mana of any color.
The Goblin dies, triggering Rundvelt Hordemaster and Mortuary.
Resolve the Mortuary trigger, putting the Goblin on top of your library from your graveyard.
Resolve the Rundvelt Hordemaster trigger, exiling the Goblin and allowing you to cast it until the end of your next turn.
Cast the Goblin from exile by paying its mana cost.
Repeat.

Combos with "Rundvelt Hordemaster"