The original Fenix was one of the most popular Twitter clients on Android, but the developer ran into issues with Twitter's token limit. That was fixed eventually, but the developer had moved on to a complete rewrite of the app. Fenix 2 launched as a preview app back in May, and now it's finished. You can buy the new Fenix right now for $1.99 in the Play Store.
Fenix 2 has a fully customizable theme, multi-account support, improved conversation layouts, and more. It feels a bit like the old Fenix app, but more modern. It's also gained plenty of features since the first preview (like a widget), in case you haven't been paying attention.
Fenix 2 is launching as a new Play Store listing, so you won't get an update to the preview app. It's a traditional paid app—no trials or in-app purchases. The $2 asking price is reasonable, too. The original Fenix still exists, but it will only show up in the Play Store if you've bought it. You can keep using it, but it won't get updates.