Some names were renown, and others were not much so. Quoting the wiki:
"The League of Legends is supervised by a council of three powerful summoners, known as the High Council of Equity. Each summoner has been on the Council of Equity since the inception of the League of Legends except for its newest High Councilor, Heywan Relivash; High Councilor Relivash's ascension took place five years ago after the mysterious disappearance of the previous High Councilor, Reginald Ashram. However, after a scandal involving High Councilor Heywan Relivash, he has been stripped of his position, was imprisoned, and has been replaced by Vessaria Kolminye. "
These names bear little to nothing of significance, however. They appeared on some champion insights or in some Judgements ( the rite necessary to allow a champion to enter league) but as far i remember they had no other major parts if not on the Kalamandra plot, and even in that case it was pretty much anonimus.
Their motivation was to simply halt magical conflicts in runeterra, or else the overuse of magic would have destroyed it. Probably some of them had hidden agendas but nothing remarkable comes out of my mind.
The power of each other was great, but more of the influential kind influential rather "raw power to shoot lazor" power. Comparing them to old Xerath is pretty much meaningless, however just know that Xerath bargained with them to access the league power so it means something.
" Seeking to rid himself of this burden, he was drawn to the magic of Valoran's nexuses and found he could absorb their power. Yet the nexuses had gatekeepers: petty mages known as summoners. Xerath knew his key to true freedom from his prison lay in gaining their trust, and he offered his power to the League of Legends. "
How they get their power was not explained - it simply was. They probably were powerful enchanters at service of nations that at some point said "ok enough".
About getting power from creatures that may vary depending on the case, but as i got it they never used them only as a "mana battery" so to speak but they bargained most of times. Some like Nocturne or Fiddlesticks were simply kept prisoniers because they were too much wild and dangerous, others like Morgana and Kayle were instead under pacts or just kept residence in the institute ( like Amumu, Ahri).
