Asterion's special blood
Asterion bleeds a mixture of red and black-- mortal blood and divine ichor, I believe. This further affirms his mortal and divine lineage, among other sources (Underworld trial, Robert's soul reading). Here are instances where this is mentioned:
- Asterion's back sores ooze a black oily liquid, and you notice dried flecks (presumable normal, red blood) (first time you meet him)
- Asterion stains his lyre black and red in one of his tantrums (just before the concert preceding the ruthless route)
- Laomedon training Asterion
to be the buff minotaur that he is(12th tablet) - Asterion's beheading (17th tablet)
Asterion's special blood also has the curious property of "hardening" or crystallizing into a 'brittle stone as if from a volcano'. I'm almost certain this is a reference to those terrifying crystals surrounding Hermes' shrine. Asterion himself tells us that it as "obsidian" when you ask him about it (a vitreous, brittle volcanic rock). The Master actually breaks off a shard of it and as he fiddles with it, leaving "oily black spots and watery red blotches".
- Blood hardening (12th tablet)
- 'Obsidian' shards (hotel's basement when Asterion first takes you there)
The basement scene in particular should have more discussion because of some interesting tidbits; Asterion intentionally introduces it as obsidian (as if hiding its identity from you), and expressly tells us to wash ourselves, get rid of our clothes, and avoid touching it further. He also describes it as "dirty", and covered in "centuries-old dust and grime". He also stands between you and the crystals after this, and wants us to spend the least amount of time there if possible. If these details aren't suspicious, what are?
And finally, some potential wrench in the theory or inconsistencies:
- In the ruthless routes where Asterion gets gored, in his sprites, you can only see red stains, and none of the black ichor. Potential theory-breaker, oversight, or intentional design. I've heard theories of demoralized no-birthmark Asterion having repressed divinity. However, this still comes to odds when you consider that he also bled black before the concert prior to the ruthless route. (Maybe because he was recollecting a memory when he was still divine?)
- In your 2nd deal with Argos, you can potentially choose Hermes' shrine with the crystals as the site of the sacrifice. This makes a great, big fire. However, the 12th tablet expressly states that the hardened blood of Asterion is a major fire hazard. I doubt Asterion (who probably knows its burning properties) would allow the sacrifice to be conducted there-- contracts and blueprints and whatnot. I'm stumped by this, actually. (Actually, maybe Asterion was confident the surrundings wouldn't catch fire or something. He also possibly burns the offering in front of Hades' shrine among a dense patch of his beloved asphodel flowers without much worry.)
Edits: Recalled Asterion's back sores, provided possible reason for fire hazard worry