They both enter at the same time, and it's only after it has entered that the ability of Cathars' Crusade can have any effect. By the time it has, so too has the mage, and you won't get to add the counter.
July 28, 2014 11:33 a.m.
I had to look the rule up on this one because I was pretty sure that it would allow you to draw the card. I was correct you will draw a card. When multiple permanents enter the battlefield at the same time they see each other enter for the purpose of triggered abilities. The relevant rule is:
603.6a Enters-the-battlefield abilities trigger when a permanent enters the battlefield. These are written, When [this object] enters the battlefield, . . . or Whenever a [type] enters the battlefield, . . . Each time an event puts one or more permanents onto the battlefield, all permanents on the battlefield (including the newcomers) are checked for any enters-the-battlefield triggers that match the event.
July 28, 2014 11:45 a.m.
@Devonin: Your response would be right if Cathars' Crusade had a replacement effect that made creatures enter the battlefield with counters (like Master Biomancer , etc.), but it's a triggered ability. Triggered abilities check right after an event happens to see if and how they trigger (which is how an ETB ability on anything is able to work at all), so objects that ETB at the same time can trigger each others' abilities.
July 28, 2014 12:08 p.m.
Rhadamanthus You still draw cards with Fathom Mage + Master Biomancer . "Enters the battlefield with counters" is treated as placing said counters so if you have Master Biomancer on the battlefield and play Fathom Mage you will draft 2 cards.
July 29, 2014 1:48 a.m.
What Rhadamanthus was saying is that if a Master Biomancer and Fathom Mage enter at the same time the Mage will not get counters because it will have already entered by the time Master Biomancer is on the field to modify how it enters. Compare this to Cathars' Crusade that has a triggered ability that happens after something enters the battlefield and will trigger when things enter at the same time as it.