When Sun Titan dies, Fungal Fortitude triggers, returning Sun Titan from your graveyard to the battlefield.
When Sun Titan enters, it triggers, returning Fungal Fortitude from your graveyard to the battlefield attached to Sun Titan.
Repeat.
This clever loop turns Ashnod's Altar into a self-sustaining engine by exploiting Sun Titan’s recursion abilities alongside the protective enchantment Fungal Fortitude. Whenever you sacrifice Sun Titan to the Altar for a pair of colorless mana, Fungal Fortitude steps in to immediately return the Titan to the battlefield. That new entry triggers Sun Titan's ability to pull Fungal Fortitude right back out of the graveyard and reattach it, setting the stage to repeat the entire cycle endlessly.