Rejuvenation is an Aura mod in Warframe that is currently only available as an offering during the current season of Nightwave. To get the mod, you will need to get season-specific Nightwave credits. This seasons credits are called Crystal Credits.
To earn Credits, you will need to complete Daily and Weekly Nightwave challenges to earn Nightwave Standing. Every 10,000 Nightwave Standing allows you to rank up, and Crystal Credits can be earned at certain Tiers up to level 30, then every Tier after that. Nightwave offerings change over the course of the event, so the mod is not permanently available.
The mod can also be traded between players, with a trade tax of 4000 Credits, so if you really need it and it is not in the rewards rotation then you may be able to get it from another player through the Trade Chat.
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It is possible for four players to run Rejuvenation mods, stacking the effect so that 12 health per second can be gained if all four players are running fully upgraded mods. The mods will also heal companions and pets, Eidolon Lures, and other friendly units such as Defectors or Hostages in different mission types. This is the main advantage of the mod, as individual Warframes can get much greater healing benefits from things like Arcane Grace, or even their own healing abilities.
The healing effect will also stack with any innate healing that a Warframe might possess, such as Nidus, and it will also stack with healing over time from Warframe healing abilities.