Phyrexian Marauder enters and dies as a state-based action due to having 0 toughness, triggering Mortuary and High-Society Hunter.
Resolve the Mortuary trigger, putting Phyrexian Marauder from your graveyard onto the top of your library.
Resolve the High-Society Hunter trigger, causing you to draw a card, thus drawing Phyrexian Marauder from your library.
Repeat.
This loop relies on casting **Phyrexian Marauder** with X equal to zero, causing it to instantly hit the graveyard as a state-based action due to having zero toughness. The presence of **Mortuary** ensures that this trip to the graveyard immediately redirects the artifact creature straight back to the top of your library. Meanwhile, **High-Society Hunter** watches from the battlefield, turning each of those creature deaths into a drawn card that effortlessly puts the **Phyrexian Marauder** back into your hand for zero mana.