Golgari Brownscale
Creature — Lizard
When Golgari Brownscale is put into your hand from your graveyard, you gain 2 life.
Dredge 2 (If you would draw a card, instead you may put exactly two cards from the top of your library into your graveyard. If you do, return this card from your graveyard to your hand. Otherwise, draw a card.)
Illustrated by
Ravnica: City of Guilds#166CommonEnglishLegality
Rulings
The first ability triggers when Golgari Brownscale returns to your hand from your graveyard for any reason, not just when you use its dredge ability.
Dredge lets you replace any card draw, not just the one during your draw step.
Once you decide to replace a draw using a card’s dredge ability, that card can’t be removed from your graveyard “in response.” (Replacement effects don’t use the stack.)
You can’t use dredge unless you’re going to draw a card and the card with dredge is already in your graveyard.