Sion is the champion whose passive allows him to revive and fight temporarily after death.
His passive is called Glory in Death. When Sion is killed, he reanimates in a decaying undead form rather than immediately waiting through a normal death timer. His health rapidly drains, but he can move, basic attack, use item actives, and cast Death Surge. His attacks become extremely fast, gain life steal, and deal bonus physical damage based on the target’s maximum health.
This effect is not a conventional permanent resurrection: Sion eventually dies again when his rapidly depleting health reaches zero. The passive fits his lore as a Noxian war hero who was resurrected after killing a Demacian king. Anivia is associated with an egg-based revival, Zac reforms from blobs after being defeated, and Karthus can cast spells briefly after dying; none matches Sion’s temporary undead combat state as precisely.