This page of the Assassin's Creed: Valhalla guide describes how to unlock the assassin's hidden blade and how to unlock stealth takedowns. These are important issues in terms of sneaking around and eliminating opponents with surprise attacks.
As a reminder, the hidden blade is the iconic weapon of the assassins, with which they are able to execute surprise attacks and eliminate their targets without alerting anyone in the area. Eivor doesn't have the hidden blade from the beginning of the game and this is easy to understand as he/she hasn't had any contact with assassins before.
The situation changes in one of the initial main quests of the game (The Prodigal Prince) - concerning Sigurd returning to his kin. A friendly Viking will be accompanied by two assassins - Basim and Hytham.
The Assassins in AC Valhalla are members of a group called The Hidden Ones. They were the ancestors of the order of assassins known from Assassin's Creed games taking place years later.
Basim and Hytham will be invited to a welcome feast for Sigurd. After the feast Eivor will receive a gift - the hidden blade.
The game allows you to immediately "test" the hidden blade - first you will be able to try different types of stealth takedowns on the hay dummies, and then use the hidden blade to deal with the enemies who sneaked near the Eivor's village.
Additional note - the hidden blade does not guarantee instant stealth takedowns in Assassin's Creed Valhalla. The target can get away alive if:
- The opponent's power significantly exceeds that of Eivor (skull icon).
- You are trying to kill an elite opponent.
The solution to the latter problem is to purchase the Advanced Assassination skill, which allows you to complete a QTE sequence aimed at murdering an elite target. It is described in more detail in the section titled Stealth.