I don't have Hiyaku I so this doujinshi was a little confusing to me. It seemed kind of...non-linear, but I'll try and summarize it anyways.

From what I know of the series, I believe Kurama is losing control of his youko half. It keeps manifesting on its own and abusing Hiei. In this series, it appears that Koenma is worried about Kurama's health (or that the youko might emerge and hurt someone) so he sends Yusuke to follow Kurama around.

Meanwhile, I believe Hiei is being held in a parallel dimension (?) whose entry way is this large, old tree. The youko keeps visiting him there. Hiei tries tricking the youko by embracing him to throw his guard off and then launching the Black Dragon at the tree. I don't think it works, though, because the next thing you know Hiei is being held naked by a bunch of branches. The youko squeezes aphrodisiac from a plant into Hiei's Jagan, which fills the fire demon with the desire for wanton pleasure.

Back in the normal world, Kurama is attacked by a demon. He tries to change one of his roses into a whip, but the magic fails. Yusuke shows up and saves him, but is perplexed as to why Kurama's rose is wilted and didn't change into a whip. I think this is how Kurama knows that the youko is causing trouble somewhere else. The next day or so, after binding Yusuke (who is still trying to follow him) with some vines, Kurama goes to see the tree that I guess links the two realms. He calls branches to impale his own body with the intention of committing suicide and destroying the youko.

It works. The next thing we see is Youko Kurama impaled by the same branches that are killing human Kurama. Hiei is released from the youko's trap of vines, and the two worlds merge. Hiei watches Kurama's human form collapse on the ground, bleeding, dying. Hiei stares at him silently, and then sits against a tree to think...

continued in Hiyaku III...