I found 2 of these sacrificial stones, is there even more Eikthyr´s to fight? Every biome has an end boss - you need to find both the spawning location and the items so summon them.
So should I start to fight him again at the other sacrificial stone place? No, unless you want to have a nice looking trophy to hang on a wall (or wherever you like) or if you need more Antler Pickaxes.
Maybe this lead to a bug appearing? The base raids are time-based (90 to 120 seconds per raid, or so). Leaving the red circle (shown on the map; the are where the raid is happening) or sleeping during a raid does not end the raid. The only way to end a raid is that the raid timer reaches zero. Enemies may or may not disappear after a raid is over - but dont ask me why and when they disappear; i just kill them all as Odin demands ;).

MFG
CKTMC