I had a question about this in reply to one of my other posts if I thought that Apollon’s role as god of cemetaries was perhaps a cthonic manifestation of Apollon. That is such a difficult question that I thought I would take a moment to discuss it in this post. Strictly speaking I don’t think that Apollon as guardian of cemetaries is what most would classify as a chthonic role specifically. It deals with death, which is natural for Apollon who governs the law of death (as Euripides describes via Apollon’s conversation with Thanatos where the latter god challenged Apollon with trying to violate his own law), but when it comes to the dead he is only dealing with the mortal remains, and the soul only during the period which it is bound to its body for the first 30 or 40 days. The soul is at this point connected more or less to the world of the living and it is during this period that sacrifices are made to the soul of the deceased. Apollon is the guardian of the tomb from violation of the living against the dead, and securing its peaceful resting place as we can infer from inscriptions from Ionia in which Apollon was invoked to protect the tomb of the dead. As a god who is, by his light, associated with the process of rot and decay, which results in the eventual seperation of the soul from its body after which it is collected by Hermes to be transported to its destination.
This is by far more connected to the role of Apollon as destroyer, as he destroys the body, and Apollon asa god of boundaries as the dead during this time cross from the living into Hades, their time under his governance. The role of Hermes and Apollon in the cemetary is a fascinating one of course, and probably not unlike their dual presence along the roadside, or even at the doorways of the homes. Apollon is the god of the road, Apollon is the god of the boundary…Hermes is the god protector of the traveler on the road, Hermes leads them forth from the boundary. As such, in the cemetary, their worship takes on two parts of the funerary process according to Pausanias. Upon the burial Apollon is sacrificed to with offerings to the dead, and then when the bonds between the soul and body decay and the soul is liberated from the rotting flesh, then there is another sacirifice to the dead but this time the god who is sacrificed to is Hermes with whom the soul proceeds into the next world.
Now whether or not the imagery of Apollon with her serpent drawn chariot has any relation to his role with the dead is unknown as Helios too has been said to have a dragon-drawn chariot which Media was said to have borrowed to make her escape from Jason after the murder of their sons. So the dragon-drawn chariot may be more of a connection to solar light. But as the sun and the earth, as we are reminded in the Homeric Hymn to Pythian Apollon in which he summons the rays of Helios against the remains of Delphinia, rots the flesh of the dead. So it may very well serve both symbolic functions in the domain of Apollon.
So is Apollon a chthonic god is the sense of other gods being such….perhaps not particularly. This is not to say that this was never the case, because the cadeceus of Hermes by which he leads the dead was once property of Apollon, but rather than the brothers share a kind of enjoined participation in the journey of the soul, Apollon dealing with the first part (which is influenced in the living world) and Hermes with the latter in the journey to Hades.