Hellcase is the biggest and probably oldest Case Opening site in the CS:GO Gambling Scene. The site is really good and lots of other sites copy their great site design. In the past many people exposed Hellcase for not having any open odds on the cases and no provably fair system so in theory they could rig all their cases in scam their users. Despite all the shady stuff, the site never really lost any of its popularity. The site offers different gamemodes, for example Case Battle, Upgrade and Trade-Up-Contracts. The different modes are all really nice and fun to play but the more you play the more skins you lose. If you really want to keep lots of skins you should just stay with the regular cases and not play Case Battle and the other gamemodes.
Gambling can cause addiction, please consider getting professional help if you suffer from gambling addiction.