I don't know shadow magic, but this seems a natural fit for psionics:

Illithid Hunger Adept

{table=head]level|bab|fort|ref|will|special|power points|morsels
1|0|0|0|2|native psionics, eat mind|4|3
2|1|0|0|3|eat knowledge|12|4
3|1|1|1|3|eat skill|21|4
4|2|1|1|4|eat power, eat spell|32|5
5|2|1|1|4|eat invocation|45|5
6|3|2|2|5|eat maneuver, eat proficiency|62|6
7|3|2|2|5|eat feat|80|6
8|4|2|2|6|eat skill trick|99|7
9|4|3|3|6|eat class feature|121|7
10|5|3|3|7|eat racial ability|144|8
11|5|3|3|7|eat power, greater|170|8
12|6|4|4|8|eat spell, greater|197|9
20|10|6|6|12|eat salient divine ability|444|16[/table]
HD: d4
Skills: 6+int
Class skills: Autohypnosis, Bluff, Concentration, Diplomacy, Gather Information, Intimidate, Knowledge(local), Knowledge(psionics), Psicraft, Sense Motive, Speak Language

native psionics (su): The only powers a IHA knows natively are Mind Link and Mind Thrust. However, they are treated as knowing powers of level half their class level for purposes of any feat or prc that requires knowing powers of a level (e.g. expanded knowledge)
eat mind (su): An IHA can eat the mind of a creature that was killed less than 5 minutes ago. This grants the IHA power points equal to the victim's hit dice, grants the IHA knowledge of exactly what capabilities the victim had in life, allows the IHA to take any specific facts from the victims mind and allows the IHA to convert the victims abilities into morsels according to the rules below. Morsels are special psionic powers. There is a strict limit to how many morsels an IHA can hold, and once that limit is reached any new morsel must displace an old one. Once displaced, a morsel is gone forever. If not displaced, a morsel may be kept forever. If the victim ever returns to life, they will no longer have any abilities which have been eaten.
eat knowledge (su): costs 1pp, grants bonus to check on a single knowledge skill equal to victim's ranks or 3+IHA's hit dice whichever is less.
eat skill (su): costs 2pp, same as eat knowledge but any skill
eat power (su): copy a known psionic power, except that the base pp cost is doubled
eat spell (su): make a known or prepared spell into a power, with pp cost 4 x level. Drop verbal. somatic and cheap material components -- retain expensive material or xp costs.
eat invocation (su): least: 4pp, lesser: 8pp, greater: 12pp, dark: 16pp
eat maneuver (su): 4pp. must have sufficient initiator level
eat proficiency (su): 1pp for 1 hour of proficiency with specific weapon, shield or armor type
eat feat (su): pp cost is twice the lowest level at which the feat could be gained. If the feat is passive, the power grants 1 hour of it. Ignore any restrictions on when the feat can be used.
eat skill trick (su): costs 1pp, otherwise like feat
eat class feature (su): same as eat feat, but class feature
eat racial ability (su): gain a special ability or attack. Cost is victims racial hit dice, or 1 if it has none. You must have any requisite anatomy (e.g. a tail for a tail-sweep). Otherwise like eat feat. You cannot gain spellcasting this way.
eat power, greater (su): like eat power, but cost not doubled
eat spell, greater (su): like eat spell, but cost is 2xlevel
eat salient divine ability like eat feat, but for sda