Demoted for losing one time at 0 LP?

your mmr was so bad that it deemed that you were not ready for that division

http://na.op.gg/summoner/userName=thedarkinlegend it wasn't one loss your mmr was quite a bit lower Remember losing games while in a party with lower elo players reduces average mmr which means losing there hurts you more and winning there doesnt do much for you So your mmr was far below the silver 1 threshold, you also were not a recent addition( i don't think to silver 1) So you had already left your 5ish game safety net So the derank was just due to low mmr and bad timing sorry to say

Demotions are based on MMR differences, rather than simple numbers. My expectation is that your MMR was slightly lower than the division you dropped from (Silver II), so the demotion happened almost instantly to try to align your visible rank with your MMR.