Martyr

Martyr is a mystical mob that rarely spawns in the Nether dimension, but is not immune to fire. Its Elytra seems to be a natural part of its body, and is used to efficiently pursue its prey.

Spawning
Martyrs can spawn out of a Ghast with a range of 1 through 20 (you can learn more about the spawning system in "More Mobs" here). You can learn more about how Ghasts spawn here.

You can also summon a Martyr using this command: /function imm:summon/imm/martyr. You can obtain a Martyr Spawn Frame with this command: /function imm:spawn_frame/imm/martyr. Make sure to replace the 'imm' portion with whatever version you're playing on if your version is older than Snapshot 3C (example: Snapshot 2A = imm2a).

General
Martyrs will pursue players and try to kill them. They will glide towards the closest survival player if they're in mid-air and continue to attack.

Martyr is technically an undead mob, and will burn in sunlight. It will also stomp Turtle Eggs and sink in water.

Also, since the Martyr is technically a stray, it can shoot Tipped Arrows of Slowness if armed with a Bow, although they can't pick one up, meaning that this is only possible with cheats. That also means that Creepers and Cavecreeps shot by a Martyr will drop Music Discs.

Jumping
Martyrs possess a high jump boost, making elevated terrain non-traversable by walking for them. They will also jump incredibly high in the air to avoid lava and fire due to not being fire resistant, but the pull of the lava current may be too strong for them to handle.

Gliding
Martyrs always glide if they're in mid-air. It works the same way as Elytras would on players, but they cannot use fireworks to boost themselves even if they somehow held them in their hands. While following a player the Martyr will glide with most accurate precision following their target. If the Martyr doesn't have a target to follow they might just glide in a straight line even if it might result in them experiencing kinetic energy.

Fighting strategies
The key to fighting the Martyr is to stay calm. When exploring the Nether it's always advised to carry decent gear and a Shield in case a Martyr shows up. If it does, make sure you are keeping it at a distance. If you want the fight to go easy you can build up until it can't reach you, but make sure the ground around has no elevations which would allow the Martyr to jump up and get to you. Good timing with a Shield and a strong melee weapon should handle it as far as there's no other external threats/distractions.

Drops
Like all mystical mobs, the Martyr must be killed by a Player to drop:
 * Corrupted Soul
 * 5 Experience

Data values
Martyrs share the same data values as all Strays (click here to view them). Some are adjusted though:
 * 1) Health: 50
 * 2) Tags: iMM, iMMNether, iMM_Martyr
 * 3) Silent:1b
 * 4) CustomName: {"translate":"imm.entity.martyr"} (§8Martyr in English US)
 * 5) CustomNameVisible: 0
 * 6) PersistenceRequired: 1
 * 7) DeathLootTable: imm:entities/imm/mystical
 * 8) Attributes:
 * 9) generic.maxHealth: 50
 * 10) generic.attackDamage: 10
 * 11) generic.movementSpeed: 0.4

Advancements
The Martyr can be required (as the Stray) for the following advancements (all of which are form vanilla Minecraft): Martyr is also targeted in these "More Mobs" advancements: Also the Martyr is indirectly related to the "Crippling Depression" advancement, as it is a mystical mob and drops Corrupted Souls which are required for the advancement.
 * Adventure
 * Monster Hunter
 * Monsters Hunted
 * Monster Fighter
 * Mob Fighter

Metahistory
In the unreleased command block version of "More Mobs" for 1.12 there was a Withered Soul mob. It was a Wither Skeleton with Elytras and a slightly different face texture. It would summon thunder whenever it spawned to notify players of its presence. Its gliding skills were very poor though. Later on this mob transformed into the Martyr. The Martyrs are not fire resistant, and don't wither you, but are still rare and have lots of health. Most of the attributes carried over but the Martyr has no connections to the Wither and the AI is improved.

Known issues

 * 1) Martyrs have a hard time using their Elytras, sometimes gliding aimlessly;
 * 2) Targeting players is unfairly precise;
 * 3) Martyrs can't avoid lava more than once.