How powerful does a demon need to be to leave a Cacodemon upon death?

Question

From the Demonomicon:

When a particularly powerful demon perishes, its soul might persist for a time as a cacodemon.

Naturally only demons with souls can undergo this:

Obyriths and transformed primordials are among the few types of demons possessing souls, as are other creatures that metamorphose into demons through force of will or through exposure to the Abyss. Lesser demons form souls through experience, through servitude to demons with souls, or through consumption of soul larvae. Demons with souls can typically reason, connive, and plan more subtly than soulless demons.

So how can I figure out whether a demon might leave behind this soul-remnant? Does the 'powerful' stipulation refer to level, or their monster role? And would I just guess if they have a soul or not in the first place?

Answer 1

Whether a demon is powerful and ensouled enough to leave behind a cacodemon is determined entirely by that demon's plot relevance.

Fourth Edition Dungeons and Dragons doesn't attempt "Gygaxian naturalism." A creature's abilities are based primarily on what seems like it will make them an interesting challenge for appropriately-leveled player characters, and secondarily on what combat abilities seem to make sense thematically. Whether they're supposed to occupy a position of power within their native ecology is not a significant design consideration.

As such, the level and monster role of a creature determines only what level of player characters it is intended to be a challenge for and the kind of challenge it constitutes in combat, not how powerful it is within the setting fiction.

This means that a GM can simply decide that a demon lives on as cacodemon if it'd be an interesting development in the campaign narrative.