It is unlocked after defeating Mega Satan as all characters.
When using the item, Isaac's head becomes like that of Satan.
Upon use, Isaac fires a giant, powerful Brimstone laser for 15 seconds while Mega Satan's roaring sound effect is heard. It is obviously a lot stronger than any other Brimstone-related items such as Shoop Da Whoop!, Brimstone, etc.
Mega Blast is strikingly similar to one of Mega Satan's attacks.