Had this issue come up the other day in a game and we could not determine a suitable outcome. Luckily we worked around it but its still interesting to discuss.
The situation is thus:
- a unit of ynnari bikers assault and kill the swarmlord, allowing them to now use a soulburst.
- at the same time, the tyranid player elects to use the "death frenzy" strategem, which states "when a character is slain they can immediately shoot or fight as per the respective phase before it is removed from the battlefield"
- as per the rules, the soulburst happens first as its the ynnaris players turn so his immediate effect rule is resolved first
The issues we had were all related to whether the swarmlord is still on the battlefield or not during the soulburst. If the soulbursting unit chooses to fight again, does the pile in and consolidate go towards the swarmlord? Is the swarmlord assumed to be removed, but then replaced in his same position for his attacks? Can the ynarri unit move into the spot the swarmlord was occupying?
As i said, we didn't really have a good understanding, but in the end i decided not to use the death frenzy strat on swarmlord and to just save the cp.