Eternal Spear

Varatha, the Eternal Spear is an Infernal Arm that can be unlocked for 4 Chthonic Keys.

Fighting style
The spear delivers repeated long-range stab attacks that can be charged to unleash a spin attack dealing high damage in a wide radius. The special throws the spear, which will damage enemies along its path until it stops. Activating the special again recalls the spear, which will deal damage on the way back. It can also be picked up manually by running over it, and will automatically return if leaving the room.

Charging Spin Attack
The Spin Attack of the spear needs to be charged before released to deal damage. It has three stages of charges:

Aspects
The Eternal Spear has 4 Aspects: Zagreus, Achilles, Hades, and Guan Yu. Like all other weapons, each Aspect has 5 levels that can be unlocked by spending Titan Blood. The higher the level of the Aspect, the more effective its improvement is. Reaching the highest level in all Aspects requires 51 Titan Blood. An Aspect will not only change the appearance of the Eternal Spear, but will also improve its attacks. The Aspect of Guan Yu has a unique move-set which changes the fighting style of the Eternal Spear. * See below for Frost Fair Blade's alternate moveset.

Revealing the Aspect of Guan Yu
Reveal the Aspect of Guan Yu by meeting the following requirements:
 * Purchase the Fated List of Minor Prophecies from the House Contractor
 * Reach the final boss (defeating the boss is not required)
 * Invest Titan Blood into 5 separate non-Zagreus aspects
 * Talk to Achilles (with any weapon equipped) until he gives you the waking-phrase ("I see you through the eyes of the crimson phoenix")
 * Equip the Spear at the House and then interact with its placeholder to reveal the Aspect of Guan Yu

To meet the Titan Blood requirement while spending the minimal amount, invest 5 into 5 different weapons with 1  each in non-Zagreus aspects (eg. Nemesis Sword, Achilles Spear, Chaos Shield, Chiron Bow, and Talos Fists). It's also possible to reveal Guan Yu with 3 or 4 weapons but this involves spending more Titan Blood (minimum of 7 for 3 weapons or 6  for 4 weapons).

Why is Achilles taking so long to talk about Guan Yu?

Achilles has a "dialogue pool" containing the conversation about Guan Yu and everything else he has a comment about but you haven't heard yet. His dialogue about Guan Yu will happen randomly from that pool, so if you have large dialogue pool it could take a while for Guan Yu to come up. If you've exhausted his dialogue options (you visit him and he has no "!") before meeting Guan Yu's requirements, he will very likely talk to you about it after the very next run.

Is it possible to trigger this conversation faster?

If you don't mind doing suicide runs (letting yourself die in the first or second rooms and talking to Achilles afterwards), you can quickly cycle through his available dialogue options so Guan Yu comes up faster. To do this quickly, reset your mirror and in the Pact of Punishment put a point into Heightened Security and max Hard Labor.

If you play normally, keep in mind that you will add to his dialogue pool if you do things that he has something to say about (like beating the final boss and advancing the storyline).