UST is an algorithmic stablecoin a.k.a. backed by the LUNA token. Hence it's pretty much almost backed by thin air; hence why Luna Foundation Guard had to purchase a good amount of bitcoin to protect the peg if it's the case that there's a bank run.
And yea, it happened:
Peg broke due to UST having a huge bank run, with people selling the UST for other stablecoins (hence pushing down the price of UST). Now for the peg to be restored to $1, Luna Foundation Guard had to liquidate a crap ton of bitcoin on their reserves just to push UST back to $1, further pushing down the downward pressure on BTC.
unfortunately the way they did it didn't work. UST didn't recover, bitcoin crashed, and LUNA died. it's over, Luna and UST can't be helped anymore. this project is dead. I wouldn't be surprised if in the next few days there will be a lot of exchanges delisting Luna from their platform.