The applicable mods for the Hellcat Power Armor are listed below.

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This currency can be accumulated by participating in public events or through purchasing from another trader, Smiley, over at the wayward shop.

Regs will sell you mod plans (which are essentially crafting schematics) in exchange for gold bullion. For context, Regs is a vendor who is unlocked over the course of the Wastelanders questline, meaning that you must complete that separate expansion before you can do any tinkering with your new gear. To purchase mods for the hellcat power armor, you will need to head over to Vault 79 and purchase the plans from Regs. Please note, figures show the cumulative stats for the complete set of armor (including the head, the helmet, the torso, the two arms, and the two legs). If you want to make up your own mind, everything you need to know about the hellcat power armor is displayed below. Meanwhile, those who are less impressed note that the hellcat suit has inferior base stats to the T-65 model that was introduced in the Wastelanders expansion. Those in favor of the new addition cite its handy special effect status, which sees each individual piece of the armor reduce incoming ballistic damage by two percent. There is currently some debate amongst the Fallout 76 community, over whether the apparel is truly superior to older examples of power armor that can already be found in the game. Once Steel Reign has concluded, you will be rewarded with the hellcat power armor and its crafting blueprint (in case you ever misplace your initial set). To unlock the hellcat power armour, players will have to complete the entire 'Steel Reign' questline.

Fallout 76 combat armor mods