Does not appear to count as completing The Dark Room or Chest for secret unlocks/achievements. Once MS defeated you will unlock one of endings(can obtain for any charater)
The wiki currently mentions that in megasatan's final phase he will attack the player with fire in normal mode. Having just completed the fight there was no point at which megasatan used a fire attack (v1.022). The final attack was a barage of large slow moving tears that broke into smaller tears (similar to the cricket's body effect).
We got screenshots of the attack in the gallery so you have you compared those? I think the reason why you didn't get to see the attack might be because you simply killed Mega Satan too fast. It's possible to skip entire pahses of bosses by dishing out damage fast enough and mega satan is on exception. --Dooomspeaker (talk) 21:13, 27 November 2014 (UTC)
The page seems to be locked against edits, but the "Currently in the game there is no reason to battle Mega Satan other than to get the last ending, as he doesn't unlock anything after being defeated." note does not apply anymore, it should be noted that it only applies to Rebirth, since you can unlock things in Afterbirth with Mega Satan
- Additionally, I feel a note should be added that if you want to kill both Mega Satan and ???/The Lamb in a single run, you must fight the floor boss before Mega Satan. There's nothing on any of those three bosses' page indicating whether you can go fight the other afterward, and with the new achievements it's reasonable to want to attempt it on a particularly good run.
It says on this page the unlocks when beating this boss can only be gotten in Hard Mode, while other pages don't say this and I just unlocked black baby on normal mode with Azazel. Seems like at least for azazel it doesn't need to be on hard mode.
The bug about chargers from My Shadow stopping Mega Satan's transformation appears to have either been fixed or it wasn't what caused that, as I just beat him with like 5 or 6 of those guys on the ground and took about 6 damage during the first phase. -- 1/26/16
odd that this page is protected; regardless, someone should add that as an exception, the game will NOT end, should a portal to the void be spawned. currently "Mega Satan does not drop a Golden Chest after beating him, instead cutting directly to Ending 16." which is correct, but outdated. -- 1/18/2016