OKAy but who are these "Sentinels" and why are they in my parking space?

Dear lore team who the hell are these 'sentinels' and how are they related to the watchers? Are they good watchers? Why did the people of the blessed isles seem to worship them/revere them seeing as they have carvings of these 'sentinels' all over the walls and floors of sacred places? wHY?

All this lore I've found out with the new map is giving me an overdose, I literally can't with you Riot.