Are there any achievement/secret for beating Mega Satan?

So I've finally collected Key Piece 1 and Key Piece 2 from the Angel Room and the golden door in Dark Room is now open.

Do I need to beat Mega Satan? I mean, I can't find any achievements that requests his defeat, but I don't know if I'll unlock a Secret after beating it.

Can anybody enlighten me?

Solution 1:

At the moment beating the fight with Mega Satan in the base game isn't linked to any "instant" achievement. Winning against him, anyway, is the only way to unlock ending 16, that is required for both Platinum God and Real Platinum God.

The Afterbirth DLC though adds 12 new achievements for defeating Mega Satan, one for each character. Each new achievement unlocks a special coop baby.

In addition to that, beating Mega Satan in Hard Mode contributes to the 12 Hard Mode completion achievements, unlocked for beating every boss and the Boss Rush in Hard Mode. Each of these achievements unlocks another new coop baby.

Solution 2:

As has been previously mentioned, in the base game, defeating Mega Satan with any character is only necessary for an ending, which is necessary for the Platinum God achievements.

However, with the Afterbirth DLC, there is now an unlock/achievement and a completion mark for each character that is earned by defeating Mega Satan. Defeating him prior to purchasing the DLC does not count for these, unfortunately, presumably because the game did not track who had defeated him.

Furthermore, there is another set of unlocks/achievements for getting all Red completion marks (Hard/Greed modes) with each character (Mom's Heart, Satan, Boss Rush, Isaac, The Lamb, ???, Ultra Greed, Hush, and Mega Satan).

All the unlocks related to defeating Mega Satan and getting all the Red completion marks are babies for second player to use in cooperative mode.