When Marina Vendrell's Grimoire enters the battlefield, its first ability triggers, causing you to draw five cards.
Shabraz triggers five times.
Resolve the first Shabraz trigger, causing you to gain 1 life and put a +1/+1 counter on it.
Marina Vendrell's Grimoire's second ability triggers, causing you to draw a card.
Shabraz triggers, causing you to gain 1 life and put a +1/+1 counter on it.
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.