I'm talking about skills like
- Sion's Enrage: TOGGLE: While toggled on, Sion has increased attack damage at the cost of some health for each autoattack. While Enrage is active, Sion permanently increases his maximum health whenever he kills a unit with either his attacks or abilities. This effect is doubled against champions and large units. or
- Nasus' Siphoning Strike: ACTIVE: Nasus' next attack within 10 seconds will deal additional physical damage. Siphoning Strike permanently gains 3 damage if it kills an enemy unit. This bonus is doubled for killing champions, large minions and large monsters
which can scale indefinitely.
Is there some comprehensive list of those champions somewhere? I was unable to find it so far.