When Marina Vendrell's Grimoire enters the battlefield, its first ability triggers, causing you to draw five cards.
Horizon Chimera triggers five times.
Resolve the first Horizon Chimera trigger, causing you to gain 1 life.
Marina Vendrell's Grimoire's second ability triggers, causing you to draw a card.
Horizon Chimera triggers, causing you to gain 1 life.
Repeat from step 5.
Notes
This combo will result in you eventually drawing from an empty library, thus losing the game. It is highly recommended to have a way to stop the combo at instant speed or to otherwise stop yourself from losing the game.