[@thewizardguy][@VitaVitaAR]

So it is definitely, indefinite. 

We know by wrote that Hakurouken will cause pain to a soul and send a free ghost to heaven, and also that Magnus is devoid of his own soul. These things are facts. 

These facts alone do not give us any further information on how the two would interact, ie. anything we decide from here is inference and assumption. So let's make our own conclusions.

First, I'll clarify what I know. Wiz might be able to help.

The 40K world is undoubtedly full of a lot of bullshit reasoning and completely made up physics and spirituality/afterlife/whatever. Daemon Magnus doesn't have a soul, so how then does he animate himself? Have free will? I have no idea. Clearly, he is something more than a ghost, and there certainly is no "heaven" in 40K, so being hit with Hakurounen won't send him anywhere. But what is he? "Warp stuff?" What is the warp? Well, that's also highly ambiguous. My impression is that it's simultaneously magic and mind, energy and soul, a parallel ever-changing universe hidden in a storm in space. Daemons and chaos gods and the lost spirits of the dead of every race in existence all inhabit it. The power for sorcery and magic come from it. The warp energy that makes up daemon Magnus is no more "soul" than it is simply pure magic essence. If anything within him constitutes "soul" it probably belongs to Tzeentch, which means Tzeentch probably felt Hakurounen hit him and now knows where Magnus is...  Oh god what have I done?

In either case, Rokurounen already hurts him, which is more than any mortal could hope for, so you're good.