A fun detail in God of War Ragnarök is that there are often unintended consequences to your actions. In this case, Yggdrasil's very future is in jeopardy, since you killed Nidhogg while trying to free Freya from her curse. This dragon was in charge of managing the roots of the tree, and her offspring are now marauding the realms. By going to see Ratatosk several times in front of Sindri 's (and ringing his bell if necessary), the latter will entrust you with the unexpected task of recovering the children of Nidhogg, without killing them: the 6 Lindworms.
You will get various rewards for each capture of Lindworm, including experience and materials. But we will especially remember the 6 sparks of Fiery Breath, which are used to improve the Draupnir Lance.
Also worth mentioning is the Everything in Its Place trophy, which is unlocked when you go to see Ratatosk in front of Sindri's house, and free the Lindworms. It is not enough to capture the Lindworms.
Position of the 6 Lindworms
This quest is very long to complete, and it is legitimate to worry a little at first, when you cannot find any of the Yggdrasil Rifts on the map. The reason is simple, you need to have advanced further in the story, so that the Lindworms rewards are actually useful to you, at least for the main ones. You must therefore wait until you have visited La Forge with Brok, to obtain the Draupnir Lance . The spear will often be used to reach the Lindworms. Rifts are always easy to find and reach, when you're in the right area.
Svatalfheim - The Forge : The first is found just after using the Draupnir Spear wall mounts for the first time, to the right after going through the crack.
Svatalfheim - Alberich's Abyss : Go to Booty Bay, on the beach to the east, near the dragon skeleton, use the spear to climb the wooden structures to access the new area. Then go up the passage in the direction of the North.
Svatalfheim - Isle of Alberich : Join the beach to the northeast, then use the spear to access the heights of the area, which will allow you to reach the small island opposite. The Lindworm is on the wooden structures between the two islands.
Svatalfheim - Le Trognon : From the entrance to the kingdom, return to the barge used to evacuate Tyr, in order to reach the exit of the mine. Take it the wrong way. New areas and access inside lead to a new fast travel portal, as well as a rift with the Lindworm.
Vanaheim - The plains : You have to access the optional area in the north of the kingdom, after defeating Heimdall and saving Freyr. Follow dog tracks. Once in the area, look in the North section of the map.
Vanaheim - The Plains : The last Lindworm is hidden a little north of the Celestial Altar, to the east of the map.
There are quite a few other Rifts to find in-game, but they contain rewards or tough enemies to face.
An earth dragon sits behind the locked door to the Forbidden Sands in Alfheim. Here's a written strategy guide with tips and tricks for defeating him on maximum difficulty, as well as a video of the fight in this mode.