What it does
Kasib Berry triggers automatically when the holder takes super-effective Ghost-type damage, halving it, then is consumed. Only fires on super-effective Ghost moves.
Ghost hits Ghost and Psychic for super-effective damage. That means Ghost types themselves have a Ghost weakness — they can beat each other up — and Psychic types are in constant danger from Ghost moves. Shadow Ball is one of the most-used Ghost moves in the current meta, and Phantom Force is a high-power physical option. The berry exists to absorb one of those.
Who runs it
Sinistcha (8% of sets) is a Ghost type that runs the berry for mirror matchups — when your Ghost runs into another Ghost's coverage. Hatterene (3%) is Psychic/Fairy, weak to Ghost, and needs to stay alive to do its support work. Slowbro (4%) and Slowking (1%) are Water/Psychic with Ghost weakness; the berry protects them long enough to execute their control roles. Basculegion (5%) is Water/Ghost and runs it for Ghost mirrors. Aegislash (2%) and Dragapult (2%) are Steel/Ghost and Dragon/Ghost respectively, picking it up for specific matchups. Skeledirge (1%) rounds out the Fire/Ghost side.
How to use it
Ghost moves bypass Normal and Fighting type immunities, meaning Ghost attackers can hit into most opponents without running into a type block. You can't just switch to a resist and solve this. The berry is how you absorb one Ghost hit that you have no type answer for.
Plan around when the opposing Ghost attacker is going to swing at your Psychic type. If you can Protect through the dangerous turn and let your partner deal with the Ghost threat, the berry might never be needed. If you have to act that turn, you're counting on it.
Vs. Focus Sash: for high-SpDef Psychic types like Slowbro or Hatterene, the berry leaves you at real HP — enough to keep functioning. 1 HP from Focus Sash is one stray chip away from fainting.
Tips & strategy
Ghost types running Kasib is a mirror-match detail that catches opponents off-guard. They calculate their Ghost move KOs your Ghost — it doesn't. You used that turn; they burned resources on a KO that failed. The less obvious your berry is before it fires, the more disruptive the timing.