Maralen of the Mornsong

• WWizards of the Coast2008-04-01
  Official Ruling
  Maralen prevents players from drawing cards for any reason, including as a cost, as an effect, and as a result of the “draw a card” turn-based action during the draw step.
• WWizards of the Coast2008-04-01
  Official Ruling
  While Maralen is on the battlefield, no player can lose the game due to being instructed to draw a card with an empty library.
• WWizards of the Coast2008-04-01
  Official Ruling
  While Maralen is on the battlefield, a player can’t choose to draw cards (or choose to have another player draw cards) as a spell or ability resolves. For example, if you cast the Development side of Research // Development (“Put a 3/1 red Elemental creature token onto the battlefield unless any opponent lets you draw a card. Repeat this process two more times.”), you’ll put three Elemental tokens onto the battlefield because your opponents can’t “let you draw a card.”
• WWizards of the Coast2008-04-01
  Official Ruling
  The effect of the triggered ability is mandatory. The life loss isn’t a payment; you’ll lose 3 life even if you have 3 or less life left. Unless your library is empty, you must put a card from your library into your hand.
• WWizards of the Coast2008-04-01
  Official Ruling
  While Maralen is on the battlefield, replacement effects that instruct a player to do something instead of drawing a card won’t work.
• WWizards of the Coast2008-04-01
  Official Ruling
  In a Two-Headed Giant game, Maralen’s triggered ability will trigger twice per draw step (once for each player on the active team). Maralen’s controller chooses the order to put the abilities on the stack.

Rulings via Scryfall