Sister Splinter is a boss in Hollow Knight: Silksong, appearing in Shellwood.
You may deal a few hits of damage to the boss by jumping into the air and smacking her before the battle begins.
With a rough estimate of 17 to 18 hitpoints in phase one the boss attempts to turn you into a shredded pancake with alternating claw ground pounds. You can temporarily stop her by knocking her off the ceiling - however, as she is heavy - if she lands on you, you will take damage. Occasionally she will summon two sharp vines which will impede your movements by limiting the arena size - you may destroy these vines by hitting them thrice with a normal attack or once with a silk spear.
Phase two, with maybe 23 hitpoints, she will begin to summon Splinterbarks along with her actions from phase one. However friendly fire is on, so she is capable of turning the spinning tops into dust herself.
Phase three with a similar estimate of hitpoints will add the occasional "vine bars". Which will separate the arena into tiny parts, make sure to break them quickly unless you fancy losing a life to a sharp log.
Sister Splinter can instantly kill her minions and break her vines, which is very useful if you bait her into it.