The Phantom is a powerful War Mode boss. It is the final boss of The End and War Mode (And as far as most people are concerned, the finale boss of the game). Being the final boss, it posses a great challenge, even to experienced players. To summon it, the player must first craft the Antidote using the six essences. Then, you must give it the guide who, once The End has started, will have become very sick (more information Here). Upon giving him the antidote, he will change in appearance. Becoming larger, with glowing eyes and tattered clothes, similar to the ones the cultist wears. He will slowly rise into the air, and the battle will begin.
When the battle begins, the player is inflicted with the "Elder's Curse" debuff which prevents the usage of magic mirrors/Recall Potions, Teleportation Potions, and mechanical items (traps, etc.). If The player travels too far away from the boss, they are pulled back with a ghostly tentacle.
The guide will start off with small, simple attacks. Like firing fast, blue projectiles, sprays of razer leaves, and swinging a spiky mace. His attacks will get progressively tougher as the player brings his health down, like summoning beams of light from the sky, firing disks that spit out thorns, and swinging an even bigger mace. Eventually, the attacks will become extremely strong. Like bringing columns of purple lightning down from the sky, summoning large, gory eyes to fight you, and firing a wall-piercing beam of molten fire and rock. He also possess the ability to redirect an attack back at the player. For example, if they player shoots a projectile at the guide, the guide can fire that projectile back at the player (however, the guide will still take damage from it).
Once the player brings the guide down to zero health, he will form a force field like bubble around himself, that will slowly regenerate his health. As he does this, a mini-invasion will occur as Dark Cultists spawn to attack the player. They spawn in a multitude of different, dangerous variants. After the player has defeated a certain number, an Dark Acolyte will spawn. It will act like a mini-Verizon of the possessed guide, with a number of magical attacks. Once you defeated the Acolyte, it will momentarily dissolve the guide's shield, stopping his regeneration, and allowing you to attack him and vice versa. He will then regain his shield, and the process will continue a few times.
Once stage three begins, a faint image of a giant monstrosity comes into existence in the background. The guide will begin to summon pieces of the monster, starting with the left arm. When a piece is summoned, ghostly practical effects appear around it as it becomes solid. While the guide summons a piece, he is invincible (the player can only damage the piece). If the player successfully destroys a piece, the guide will begin summoning a different one. If the player does not destroy a piece, the guide will begin summoning a new piece, but the player will have to cope with the powerful attacks of completely summoned piece. As he does this, the guide will be periodically using attacks he did in the first stage.
If the guide's health is once again, brought down to zero, a white flash will cover the screen as multi-coloured beams of light shine from within the guide. He will then collapse on the ground. You can talk to him again, but the only thing he will say is, "I knew this day would come. Do not mourn for me (name of player), for I have created a hero." With that, the guide will fade away. A message will soon appear saying:
"Goodbye, (name of player)"
With that, the background turns to black, the edges of the screen darken, and the ground shakes. The Phantom will then spawn with the message, "The Phantom has awoken!".
The Phantom looks like a reincarnated, almost zombie like Moon Lord. However, it is much larger and has four arms. The Phantom also has skeletal wings protruding from it's back. Along it's body the Phantom has six glowing lights embedded in it of different colors. These orbs act as weak spots for the Phantom, and are the only points at which you can damage it. Being the final stage, this fight requires all the players skill; but still allows for a variety of fighting styles. The Phantom has pattern of attacks (much like older video games) that the player must memorize if they want to avoid getting killed. The Phantom's attacks include firing a lime green laser from its center eye, creating huge air currents with it's wings, moving it's hands and arms in different ways (clapping, slashing, grabbing, etc.), shooting fiery projectiles from it's hands, and summoning strange monsters (similar to the True Eye's of Cthulhu). As the fight progresses the phantom will fire a shadowy project at the ground that, for a time, removes the ground causing the player to fight as he/she free falls. Along with these height damage, special attacks, the Phantom has an almost uncountable amount of smaller attacks similar to the guide. These include creating a defensive ring of projectiles around itself, causing the ground to sake a deal damage, and flying around in crazy directions forcing the player to follow. Unlike the earlier stages, the final stage of the fight is timed. When it begins, the background begins to light up as the sun starts to rise. If the sun does appear before the Phantom is defeated, it will shoot five golden beams (one from each eye) at the player. It is impossible to avoid these beams and they will instantly kill the player. If the player destroys all the weak spots, another bright flash of white will cover the screen and the message, "We will return, we always do." appears. Then the screen fades to black, and the player will find themselves at their spawn point.
At this point, much has changed in the world. The spread of the Corruption/Crimson and Hallow is reverted back to what it was in pre-hardmode. Monster and event spawn rate is decreased. NPCs will congratulate the player on his/her defeat of the Phantom. Also, the player will occasionally find the Herald who lets the player fight Champions. You are now able to craft the Pentagram Stone to start The Fallen Ones Event