When any coin blockchain ( ETH, Bitcoin, ETC, Verge) is attacked by any miner group which can control more than 50% network mining of that particular blockchain results in 51% Attack. Mostly miner group used the rented hash to attack any coin blockchain. It's not free of cost to attack any Blockchain.
what can they do after that?
That miner group can stop any transactions on the network to get confirmed and can even reverse the transaction which is completed at the time of control. They can interrupt the generation of the new block means if any other miner is mining the block they can stop it to get confirmed and earn the reward of that miner block themselves.
What they can't do?
By controlling the mining power above 50% doesn't mean that they will control that particular blockchain forever, they cant forked the coin by getting control of mining hash power or bring changes to old confirmed blocks transaction data. The time they have control they will keep getting the block rewards.
History of 51% attacks
It's not like that any blockchain is safe from them but yet BTC has not suffered any such attack however, Eth has suffered an attack in 2016. Bitcoin Gold also suffered a 51% attack twice which was in 2018 and then 2020. Verge also suffered from attacks in 2018 and lost 20 million coins. some other coins like feather coin, vertcoin also suffered a 51% attack in the past. A recent attack during this month of august was on Etc which was attacked a second time in a month of august and overall third time in a year lost millions of dollars worth of the coin.
BTC is one of the known and most used cryptocurrencies in the world. China controls more than 50% of the mining power but any attack on the BTC blockchain will cost very much to any miner thinking of attacking BTC Blockchain. It is estimated that more than $682,993 will be cost to attack BTC blockchain for 1 hour. A serious working is needed by companies that still most of the blockchain can be attacked at a lower cost.
This article was also shared on uptrennd platform by me.