Haven't read the lore for eternatus.Regardless, this reminds me of the ultrabeasts, which frankly were the MOST hideous designs for Pokemon so far (granted they are "extra-dimensional") but Deoxys the alien was an amazing design IMO. Necrozma, had one thing, the name was cool, other than that it was my least favorite legendary due to its hardly recognizable/difficult to discern design. Again, eternatus falls into the SAME EXACT RYTHM. It is a horrible design, when I first saw it I was pretty upset (maybe the spiral looks kinda cool). Still, every time I see it I have to tilt my head to grasp the proportions and "look". God this is bad.
On the other hand... Zacien and Zamazenta are some of the coolest designs for recent legendaries. Nice, simple design (for the unarmored forms), still pretty cool when armored, Well Zacien looks better armored, Zamazenta vice versa. Also, the twin wolf lore connecting with UK history is a great addition, along with the "stone henge" Pokemon with the Ingwaz's on its legs (even though it's not the best design honestly, but it's ok), Impidimp line (goblin/troll folklore), etc.
