Athames (Charmed); used by warlocks to steal powers
Dark One's Dagger (Once Upon A Time)
Marceline (Adventure Time) stated the only power her demon heritage granted her was the ability to suck souls, like her father. This allowed her to gain the specific abilities of vampires she had killed.
Ergo Mundus (Anima: Beyond Fantasy) can absorb and assimilate the fundamental conditions of any artifact or entity he destroys, gaining partially or fully their powers and skills, being able to use them as their own.
The High Arbiter Aizen (Anima: Beyond Fantasy) absorbs the power of all those he has defeated.
Reinhard Tristan Eugen Heydrich (Dies Irae) can use the abilities of everyone in his Legion as well as those that are killed by his Legion or his lance, Longinus Testament.
Immortals (Highlander) can absorb the knowledge and power of any Immortal that they defeat and behead in single combat, a phenomenon known as the Quickening.
Upon defeating the Robot Masters at the end of each level, Mega Man (Mega Man series) is able to obtain their signature attacks for himself.
Steel Inquisitors (Mistborn) as hemalurgists, gain their powers through killing a Metalborn person with a spike and implanting that spike in their own bodies, taking on a weakened form of their abilities that depends on how long it has been outside of blood or a body.
Only with the Dark One's Dagger (Once Upon A Time) can the Dark One be killed, the name of the killer appearing on the dagger's blade, granting them the powers and title the Curse of the Dark One onto the user.
After killing Whitebeard, Marshall D. Teach/Blackbeard (One Piece) gained his former captain's Tremor-Tremor Fruit/Gura Gura no Mi power.
Cinder Fall (RWBY) absorbed the power of the Fall Maiden by killing Amber, the previous Fall Maiden.
The Hive Cleaver, signature weapon and illustration of the Sword-Logic.