Where necromancers of the school of necromancy of Ithron cast this spell it will always be to send a soul either to Kharach or their own god respectivly, or just straight to Kharach. If no direction is given, Kharach is the default destination, souls cast directionlessly seem to be drawn to the halls of Kharach.
However, it is possible to attempt to direct the spell to other places in other planes, for example, Sharda or the Utterdark. I speculate that the more evil practitioners of the dark arts could use this spell or a variation to feed souls to extraplanar entities other than Kharach but I would not recommend anyone with a charged scroll attempt this in any way or for any reason. It will end badly for them.
As a footnote, this is is also why you always bumrush witches or demons casting rituals over dead or dying bodies with urgency.
I hope this information is helpful.
Rattle of Clan Dingwall
Master Necromancer, Former apprentice to Exodhan Pyre