Sacrifice Flayer to Altar, adding {C}{C} and triggering Undying and then Flayer's damage trigger.
Pay {2} to activate Sage of Fables, removing the counter from Flayer and drawing a card.
Note you may hold priority while the Sage of Fables draw trigger is on the stack and continue the combo without drawing cards to avoid decking.

Infinite card draw

Infinite damage

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Ashnod's Altar

Flayer of the Hatebound

Sage of Fables

Activate Ashnod's Altar by sacrificing Sage of Fables, adding {C}{C}.
When Sage of Fables dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Activate Sage of Fables by paying {2} and removing the +1/+1 counter from it, causing you to draw a card.
Repeat.

Infinite draw triggers

Near-infinite death triggers

Near-infinite creature ETB

Near-infinite creature LTB

Near-infinite creature sacrifice triggers

Mikaeus, the Unhallowed

Activate Sage of Fables by paying {1} due to Training Grounds and removing a +1/+1 counter from a creature you control, causing you to draw a card.
Lyla triggers, putting a +1/+1 counter on Berta.
Berta triggers, adding {1}.
Repeat.

Berta, Wise Extrapolator

Training Grounds

Lyla, Holographic Assistant

Activate Phial of Galadriel by tapping it, adding {1}.
Activate Sage of Fables by paying {2} and removing a +1/+1 counter from a creature you control, causing you to draw two cards.
Vodalian Wave-Knight triggers twice, putting two +1/+1 counters on each other Merfolk and/or Knight you control.
Activate Mind over Matter by discarding a card, untapping Phial of Galadriel.
Activate Phial of Galadriel by tapping it, adding {1}.
Activate Mind over Matter by discarding a card, untapping Phial of Galadriel.
Repeat.

Infinite looting

Infinite self-discard triggers

Near-infinite +1/+1 counters on certain creatures

Mind Over Matter

Phial of Galadriel

Vodalian Wave-Knight

Activate Sage of Fables by paying {2} and removing a +1/+1 counter from a creature you control, causing you to draw two cards.
Vodalian Wave-Knight triggers twice, putting two +1/+1 counters on each other Merfolk and/or Knight you control.
Activate Skirge Familiar twice by discarding two cards, adding {B}{B}.
Repeat.

Skirge Familiar

Combos with "Sage of Fables"