The rules for chaos boons states you must do it immediately if the champion kills a character.
if a model is reduced from 1 to 0 wounds it is removed as a casualty.
models removed as casualties are removed from the table and placed to one side.
the spawn/
DP model has to be placed within 3" of the champion of chaos model, if its off the table you can't really do that.
so the question, as most of us already know, is which happens first.
as codex generally trumps rulebook, and the rulebook does not say immediately remove the model, but tells us we must immediately roll on the boon table. Because part of being reduced from x-0 wounds is being removed as a casualty which is going off the table and its not listed a separate step the question is now does "must immediately" interrupt the process. Technically both models are removed at the same time so when you kill the model with the champion it kills you at the same time. We cannot say this is an event that can be sequenced, or the order picked by who's turn it is, because we are told:
If both sides have models with the same Initiative, their attacks are made simultaneously.
so unlike other times when we are not told how to resolve things that happen during the same part of a phase, we are spefically told attacks are made simultaneously here.
as such we cannot invoke a sequencing rule and say that we can pick the model to be removed before the champion is removed.
The result is the champion is removed at the same time it kills the other model s the champion is definitely removed from the table.
Despite it seeming illogical to roll a result on a table for a model that is dead there is nothing in the rules stopping it from rolling on the boon table, as there is no requirement in the rules for the champion to be alive, at full wounds, or on the table to roll.
As such you can still roll on the boon table, but a result of spawnhood or daemonhood does nothing as the champion is off the table and you cannot place the spawn daemon within 3" of the champion.
Under the spawn rule, which daemonhood says to follow as well, we are told that if the spawn/daemon model cannot be placed on the board it counts as killed.
The only issue with the above is if you roll "bloated' which gives you +1 wound if at full wounds, or is returned to full wounds. Technically the champion is alive again but is off the table as it was removed as a casualty.