The "Refreshment" perk in The Witcher 3.
It does require a bit of leveling up before you can unlock it so you can't get it right away but especially with the latest upgrade, you can actually equip it by the time you're done with the Bloody Baron (you need to invest 6 skill points into other alchemy skills first).
This perk gives you more vitality (health) when you drink any potion. Since you can drink potions during combat, and replenishing potions is super easy with the game's much simplified alchemy system compared to previous games, and you can quick slot potions, this basically makes you invincible as long as you're facing enemies at your level.
For example, quick slot the Thunderbolt and Tawny Owl potions for strength and stamina acceleration- the first two potions the game gives you for free- and pop them while fighting and you're stornger, can cast spells faster, and at full health all the time. Now you broke the game.
Update with the "next gen"... er.. update..
They made getting to those higher level perks easier. Each level within a tree requires having had already invested a certain number of total points in that tree to unlock them, and now they made it require less. They also lowered the amount of possible upgrades for some, to balance.
So before the next gen update, you would need I think 8 points to unlock Refreshment IIRC, so I would put 5 in Poisoned Blades and 3 in Acquired Tolerance. That means I wouldn't get to Refreshment until well into ore maybe after the Bloody Baron questline, which is the hardest part of the game anyway.
On the next gen play-through. you only need 6 points to unlock Refreshment, and none of the skills allow more than 3 upgrades anymore (but the 3rd is as strong as what was previously the 5th so nothing is sacrificed). These changes allowed me to unlock Refreshment before even starting Velen.
An additional bonus is that you get green mutagens first, which gives you an additional vitality bonus if you equip green skills.
So- you get to Velen, you have a green mutagen, Poistoned Blades, Refreshment, and Frenzy (to slow down enemies when you drink potion), and you're already practically invincible.
