A friend of mine is playing around with the Death 9 blessing, and got some odd results:
1. Sacred units killed while blessed return as mindless undead. (This isn't so odd but I'm pointing it out for people who don't know what Death 9 does.)
2. However, if you have no commanders with undead leadership present, they continue to fight normally. They don't dissolve like mindless undead should. (These and the following points discuss non-commander sacred troops.)
3. Afterward they appear in the organization screen in the same squad they were in when alive. However, they can't be assigned to a new squad under the same commander (because he can't command undead). You can assign all the living members of that squad to a new squad though...
4. In subsequent battles they still fight normally (even under a commander who can't command undead, and with no commanders who can command undead present).
Finally, and just for humor value: if you're doing this with Abysia, watch out, your heat auras can set your reanimated troops on fire. (Even that doesn't rout them, though - they're mindless.)
Frankly, the stats on the reanimated troops seem bad enough that I don't think Death 9 is particularly impressive compared to the other level 9s. It seems to me that you should at least get something reasonably powerful (about wight or lictor level) - although that would be pretty impressive if you started with flagellants,
most sacred troops cost 30 and up each. Getting a soulless doesn't seem like much of a consolation for losing them.