Like @moon827 said its mostly effective on bruisers, because you have sustain and damage. With assassins you go in burst then go away. You need to keep the fight going on for more than 4 seconds, you also need to make are you can survive in a fight for that long. It basically Grasp of the Undying without the healing and health increase but dealing extra damage instead. pairing Conqueror up with Overgrowth on bruisers could be effective without losing the maximum health increase.
Sounds pretty terrible on a jungler. A rune that takes 4 seconds to stack does literally nothing in a gank. It's made for toplaners who can stack it on creeps. IMO it's really for splitpushers who want to duel tanks.
Conqueror could definitely work but there are many situations where it is outclassed.
For example, Warwick and Udyr would prefer Press the Attack in ganks. Both champions easily get three auto-attacks in succession and the % damage boost applies to all incoming damage, rather than just your own damage, which is beneficial in ganks.
I've not done calculations on damage output, but assuming runes operate best under their focus, Conqueror would be best into duels, or a jungler playstyle of solo early invades and split-pushing.
Hope this helps,
~moon
For example, Warwick and Udyr would prefer Press the Attack in ganks. Both champions easily get three auto-attacks in succession and the % damage boost applies to all incoming damage, rather than just your own damage, which is beneficial in ganks.
I've not done calculations on damage output, but assuming runes operate best under their focus, Conqueror would be best into duels, or a jungler playstyle of solo early invades and split-pushing.