Sentient artifacts and Thresh's lore.
There are a few champions that exist because they were originally artefacts who would find and corrupt their hosts.
is now host to something that increases his rage, anger and violent tendencies particularly towards the aim of vengeance and revenge. This host, however, as far as we know, has not spoken to anyone but Varus himself.
's body is was human, particularly a sea-farer. The artefact that corrupted him was underwater in a cave a fire that can't be quenched. This artefact has taken complete (100%) over its host's body. No doubt this being is completely sentient unlike the previous example of Varus.
was clearly corrupted by something, may or may not have been an artefact but definitely a malevolent being. His intentions are quite somber and vague but he essentially brings destruction not because of a clear goal, just because he's angry he got stuck in a stinky fishy suit. This corruption source doesn't seem to have talked to anyone, not even Nautilus, might not even be sentient at all.
's corruption source almost seemed sentient but since it was an artefact I will include it here. His artefact is closely similar to Varus, the main difference is that Varus is urged and influenced by his corruption. Zed, it seems, only gained immense power from his artefact. Zed's artefact may or may not have amplified is diabolical nature. Overall Zed seems to not be influenced by any outside sources.
The reason why I bring these up is because of Thresh's new, amazing, lore where it mentions that in life Thresh was a part of an order whom kept these artefacts locked up, separate and contained so that they may never haunt and ravage the outsiders ever again. One particular passage of this lore mentions that Thresh enjoyed torturing the sentient beings in the artefacts that was under his watch. I'm wondering how many of these would have existed to corrupt and influence hosts, similar to how Varus and Brand was corrupted by theirs. What would these artefacts have been? What would have been their form as a parasite inside their host?