This is my attempt at making a Ranger subclass relevant to the Witcherverse by Andrzej Sapkowski.

For all future references, a monster is any creature that is not a humanoid.

Bestiary

Your training in the ways of hunting monsters has made you an expert in recording and extrapolating data about the habits and lifestyles of many monsters, as well as the knowledge regarding how to fight them. After you kill a monster, you may take 10 minutes to record it in your Bestiary before leaving the area. Make a Monster Knowledge check, with a DC equal to 10 + the monster's CR - the number of times it is listed in your Bestiary. If you succeed, you learn one damage immunity, resistance, or vulnerability of the DM's choosing each time you study a corpse.

Witcher's Training

When you choose this subclass at 3rd level, you gain proficiency with the Herbalism Kit and the Poisoner's Kit. You also gain proficiency in a new skill, Monster Knowledge, a Wisdom-based skill that allows you to figure out lore and weaknesses of monsters.
Additionally, longswords count as Finesse weapons for you.

Witcher Sight

At 3rd level, you gain darkvision out to a range of 60 feet. If you already have darkvision, its range is increased by 30 feet.

Signs

You gain access to additional spells at 3rd, 5th, 9th, and 13th levels. Once you gain a Sign, it counts as a Ranger spell for you, but doesn't count against the number of Ranger spells you know.

3rd - Igni (Fire Bolt), Aard (Mind Sliver)

5th - Quen (Shield)

9th - Yrden (Slow)

13th - Axii (Confusion)

Witcher's Senses

At 7th level, you have advantage on Investigation, Perception, and Survival checks to track monsters that are in your Bestiary.

Witcher's Constitution

At 11th level, you gain proficiency in Constitution saving throws. Additionally, you can add half your Constitution modifier (rounded down) to any hit dice you expend during a short rest.

Monster Slayer Attack

Beginning at 15th level, you can attack thrice, instead of twice, whenever you take the Attack action on your turn against a monster in your Bestiary.