Bloody Mary

Bloody Mary is a challenge added in Antibirth, unlocked when Bethany, the Blood bag and It Lives are all unlocked. And unlocks the Rune of Gebo upon completion by defeating Satan.

The run starts with Bethany (three hearts and four soul charges) with:

Blood Bag (+1 Heart).

Anemic (+5 range and creates creep upon getting hit).

The Book of Belial (A three charges active item which increases damage on activation and passively increases Angel/Devil room's chances of spawning upon completing a floor).

Blood Oath (at the start of each floor increases stats at the cost of leaving you with half a red heart).

Child's Heart trinket (Increases heart pickup drops)

Description
By looking at the starting items you see immediately where all the difficulty comes from. Since Bethany cannot use soul and black hearts as health, at the start of every floor (except the very first) You'll be always dropping to half a red heart and you must search for healing items and pickups through the floor. The Child's Heart helps, but you may not rely entirely on it.

Strategy

 * You must be very good at dodging to survive the meantime you are refilling health or get lucky with protective items like the Holy Mantle, Bot Fly, Saturnus, and orbitals like Big Fan, Tinytoma and Cube of Meat.
 * health increases not only the maximum amount of hits you can receive but also because the more health you have at the start of the room, the more Blood Oath increases your stats.
 * Items which regains health like Placenta and Leech.
 * Another Blood bag will fill you five hearths and add another one, but unless you got some invincibility, it would be hard to get from the Donation machines and you may get the IV bag instead
 * Yum Heart will exchange two soul hearts for one red health, making it useful as a last resort to regain health at the cost of the Book of Belial.
 * Dark Bum will turn red hearts to soul hearts, meaning more charges, but if you aren't fast enough he will leave you without any health to work with
 * If you have trouble keeping yourself alive due to the health, you can bypass the Blood Oath by leaving the room before it stabs you (via Telepills or the door if you are fast enough), but it won't give you any stats increase.
 * At least at the run's start, you should avoid deals with the Devil since the less red health you have, the less the Blood Oath rewards you and you can receive fewer hits before dying; and the rewards in soul/black hearts won't be that helpful unless you change the Book for a more spammable item like Tammy's Head
 * Since Book of Belial uses three charges instead of the regular two from the Book of Virtues, you may be cautious about how many soul/black hearts you have and will spend on activating the item (One heart equals two charges, so you need one and a half soul hearts to activate it in case the item charge is depleted).
 * Note that the damage increase can be activated again in the same room, but will be lower than the first.
 * 9 Volt will save you a charge and lower the cost to one heart.
 * Except for the increase in range, you won't find Anemic very useful unless you got a lot of health afterward or use the Voodoo pin.
 * Avoid Magic Skin since it will sacrifice a health slot you may want and will replace items afterward with itself which could be health, damage, or speed upgrades.
 * Luck stat implies more pickups chances on clearing floors and opening chests and more pickups means more chances for red hearts to spawn so items that increase luck are good.

Trivia

 * The challenge's name may refer to the Urban Legend and ritual of the same name, which depicts her as a ghost described as covered in blood, who will (among other things), drink the participant's blood.