You have to attack the Natar village that holds the artefact with hero in the attacking army and win the attack. You also must destroy the treasury in that village. You also have to have treasury required by the magnitude of the artefact in the village you attacked from: Treasury level 10 for village artefact, and level 20 for account wide and unique artefact.
The artefacts in Natar villages are always in Treasury level 20, no matter the magnitude.
Normally artefacts are heavily defended, however the Natarian defence is different on each server and is set up based on the top 100 offence armies of the game world.
HINT: Villages that have an artifact in them, cannot be destroyed to vanish from the map. That protection will lift when/if the artifact is moved or captured.