Mother's Shadow

Mother's Shadow is a mini-boss found in a sub-area of the second floor of Chapter 2.5 ( and ). It spawns on the way back after obtaining the. It's completely invincible and only serves as an obstacle in a chase sequence.

Behavior
After entering the sub-area, all of Isaac's items, pickups, and trinkets are stripped away, leaving him similar to his state during the start of the game. He must walk through a couple of empty rooms before reaching the end room containing. Picking it up causes the room to shake and summons Mother's Shadow.

Mother's Shadow, after appearing, begins slowly floating after Isaac. When close enough to him, it stops and then quickly dashes at Isaac, dealing contact damage at all times. Isaac can 'push' Mother's shadow before its charge however and will not take contact damage. Mother's Shadow can go off-screen with its dash attack or when a door is crossed into a different room. It then resumes following Isaac.
 * Mother's Shadow's position at all times is determined by its position relative to the general layout of the floor, meaning it persists in the same location even when Isaac moves through rooms. However, it cannot be left completely behind and is carried along as the screen moves.
 * After encountering Mother's Shadow, the previously empty path back to the main floor map becomes filled with a variety of obstacles, some of which must be traversed past using Throwable Bombs from Bomb Grimaces, buttons, and sometimes keys and certain cards that spawn in each room.

As soon as Isaac passes the door leading out of the sub-area, Mother's Shadow screams Isaac's name in defeat and ceases to appear again, as the sub-area's entrance collapses and becomes blocked by falling rocks. It is no longer possible to enter the area again past this point.

Strategies

 * The key to avoiding damage during the chase sequence is to be patient. Trying to rush to the exit door will often result in unavoidable hits.
 * Mother's Shadow will not attack Isaac directly between dashes, only move in his general direction. She will scream right before attacking, and will lock its direction; stay in place until she starts to dash and swiftly move out of the way.
 * Going back into the previous room doesn't cancel its attacks, but provides more space for avoiding them, as well as invincibility frames.
 * It is strongly discouraged to stand on one tile-wide paths for too long, because they limit Isaac's ability to move out of the way of its dashes.
 * Since Isaac keeps any health he enters the area with, it is wise to stock up on hearts beforehand to minimize chances of death.
 * ,, , and can escape with minimal risk, as they can fly directly to the exit door.

Trivia

 * The behavior and role of Mother's Shadow is similar to that of Phanto from Super Mario Bros. 2. In that game, it guards Keys and chases the player relentlessly if they pick one up. It only stops its pursuit if the Key is dropped or used to open a door.
 * It is also potentially a reference to Pamela Voorhees in the Friday the 13th NES game, as both are flying heads with messy hair, and both attack the player by charging directly at them.
 * If a Crawl Space or Black Market spawns, the player can regain access to their items and cards/pills/runes in that space; however Mother's Shadow will continue to give chase. If the player progresses to the next floor while in this space, Mother's Shadow dies, letting out the Isaac scream and temporarily locking the doors at the beginning of the next floor.
 * Mother's Shadow resembles the face of Mom as she is depicted on the cover art for Repentance.
 * Sometimes when charging, Mother's Shadow can yell Isaac's name in a deeper voice.
 * The only items that are not taken away from Isaac in this sequence are, , and.
 * Blue Flies, Blue Spiders, Locusts, spawned from a, and Wisps spawned from  persist into this sequence but have no effect on Mother's Shadow. Blood clots spawned by  will also persist and are immune to Mother's Shadow.
 * Although Mother's Shadow has 250 health, it can not be damaged or killed, even with.
 * Using  while entering the room containing  will kill Mother's Shadow before the chase activates. Picking up  will not start the chase if Mother's Shadow has been killed this way.
 * Due to how the escape sequence is spawned (one version spawned when you enter for the first time, and one edited version spawned when Mother's Shadow appears after is collected), items relevant to the escape sequence, such as cards used in puzzles like, will spawn both when the area is entered and when the escape sequence itself begins. Ergo, despite normally being blocked off, if you have some way to collect these items on the way to collect  (such as methods either as simple as using a character like  or  and their innate ability to fly, or complex ones such as using the explosive tears made by Wisps spawned from  combined with  to break rocks that prevent items like these from being obtained on the first run through), you can obtain an extra copy of these items, allowing you to 'skip' rooms that may not account for you possessing these items before the escape begins - for example, using the aforementioned  within the room you got it from before Mother's Shadow spawned to simply fly through the room, grab another copy of the same card spawned by Mother's Shadow's appearance before exiting, and then use that copy in another room.