Knowing most item effects is very important for Apollyon, as you want to know whether or not to absorb something.
The Battery and Charged Baby allow Void to be used twice consecutively without going into other rooms. This is helpful if you want to use Void to fight the boss and then absorb the item after defeating it after all rooms have been cleared, or when a Devil/Angel Room spawns after defeating the boss and you do not have Goat Head or Eucharist.
There's Options and More Options are amazing items for Apollyon, as they give the opportunity to absorb twice as many items that could increase his stats or add to his actives collection.
Absorbing passive items from a room with choice pedestals will count as absorbing only one of them, this includes pedestals added by There's Options and More Options. Choice rooms can still be effective for copying active item effects, however.
Void can be used during the 'item get' animation to absorb the held item. This allows Apollyon to absorb undesirable devil and shop items, albeit by paying their cost first. It can also be used on items formerly unidentified due to the Curse of the Blind (Pausing is recommended right after picking up the item to properly check what it is before absorbing it or not).
Finding a Library can be very powerful as Apollyon can use Void to gain numerous amounts of active effects from one room.
However, it is not recommended to absorb a few books for the following reasons:
The Bible will instantly kill you if used while fighting Satan. However, taking it is fine as long as you don't forget you have it or aren't fighting him.
Satanic Bible replaces the boss item with a Devil Deal, which could be bad if low on health. Devil deals also cannot be absorbed until they've been paid for.
How to Jump causes a sudden movement, which can potentially throw you into an enemy or trap you behind rocks (the latter can be fixed by leaving the run and continuing).
Book of Virtues is treated as a passive item if absorbed by Void. It's much better to take Book of Virtues and gain the passive wisp effect over a minor stat boost.
It's also a good idea to touch at least three books if possible before using Void, to get the free Bookworm transformation.
Birthright: Using Void has a chance to spawn an item previously destroyed by it while keeping the bonus stats. The more items absorbed into Void the higher chance for an item to spawn upon activation. Has no effect if Void is not held.
Apollyon is the Greek name for the angel Abaddon, and is also known as "The Destroyer". This is reflected in the Achievement pop-up, which states "You Unlocked The Destroyer".
This is referenced further in the Void item he starts with, in that he is both the angel of the abyss and that the main function of the Void is to destroy items to gain beneficial effects.
In the New Testament Book of Revelations, Abaddon is written as the king of an army of locusts, hence the locust trinkets he unlocks.
In a Twitter Q&A, Edmund revealed details about Apollyon's physical appearance, stating that he is a living statue, and has flesh but it appears like stone. He also confirmed that Apollyon has no eyes but can see out of the holes.
In the original Afterbirth+ release (before Booster Pack #5), Apollyon was unlocked by default (except in the Switch version).
Even though Apollyon appears to have wings, he does not start with flight. Edmund said he has "tiny, useless wings".
Apollyon's locked character icon is a locked box with dark energy swirling around it