Hello I'm Netechma!
You need to find the coffin of Zi'Assunta Belpaese in Cappella Magna, the fifth stage (unlocked by hitting level 80 in Gallo Tower)---------------------------------------------------------------------------------------------
Upon starting stage head straight down south verrry far to the white question mark symbol (on your map when you press start to see it)
Break the weird fence thing with an attack.
Defeat the enemies surrounding the coffin and walk over the coffin itself to open it and unlock the character.
The achievement itself will unlock after the stage ends if you met that criteria.
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*edit thanks to Danthewhale for the info
DanTheWhale
Okay so I figured out what's going on, the blue reaper spawns in once 5:00 min have past on the map so you really need to hurry and get to the coffin before they spawn in otherwise you'll need a pantagram to deal with it.
For anyone having trouble with the Reaper I used Sigma but Crapher suggested:
There's also several rosaries on the map as you make your way down, in the middle, which will insta kill that blue reaper. Just go to the furthest one you can as you level up and once he's in range pick the item up to kill him. Gives you time to level up a bit to take on the enemies around the coffin too.