On this page of the Valheim guide, you will find information on how to get Ancient Seed.
Where to find Ancient Seeds?
You can get Ancient Seeds in three ways. The first is to fight Greydwarf Brutes and Greydwarf Shamans. When killing these opponents, you have a chance that they will drop an Ancient Seed. These creatures are common and easy to encounter even at night in Meadows.
The second method uses the so-called Spawners. These characteristic points emit a purple glow a spawn certain types of enemies. If you destroy an object of this type responsible for spawning Greydwarf Brutes, you will receive an Ancient Seed.
The last method of obtaining it is to search the chests located in ruined houses. There is a chance that seed will be in one of them.
What are Ancient Seeds for?
Ancient Seeds are needed only in one specific moment – to summon The Elder, the game's second boss.