Had been summoned here by PM, here I come. Pighead In, let's take a look at your idea.
Elevator Pitch : So, it's a pretty good start and it's an anomaly who is not underdeveloped, which is already good. The only problem I have with this one is that we don't know much about them and, mostly, about their goals and why they act the way they act. Why we can access the dimension only if we put the bucket over our heads? Why the challenge and why the entity wants to challenge them? Why the people who lose are decapitated and why they put the hermit crab as a "replacement" (we can say it's a replacement)? Why the entity wants other people to wear the bucket next? Here's questions that I recommand you to think about. For the rest, not a bad anomaly at all.
Central Narrative : Again, not bad at all but I'm not totally fond of all, because it's not too original and it's something that I can see sometimes. I've already seen these stories of new researchers who use the recent discoveries of SCP objects in order to prove that they are worth to be considered as skilled scientists. And even if these ideas can or cannot work mostly because of their execution and are not automatically bad, I don't know. I'm a little bit skeptical, I guess.
Hook : OK, so I think you have a good hook. The fact that the entity is cocky and cannot stop talking is a good opportunity for you to tell your readers about his goals, about why he do what he does and knowing more about his goals. You said that the creatures are looking for a new home because their previous home is unsustainable : explain that, let the entity talk about that and about the reasons of why their home is unsustainable!
And about your character, try to avoid the extremes. If he come back with more qualities than before, like "Now, he's stronger, more wise and became super useful about everybody and super resistant", it will not work. But if he also come back like "All that shit makes him mad and now, he's probably dangerous and everyone needs to be careful", this will be even more cliche and less interesting.
And I will stop here. For the moment, I will not greenlight. I feel a lot of potential on this idea and there's things who are pretty interesting, but I want you to work and think about all the questions and paths I've put in my post, this could help you to make your skip better! For the rest, don't hesitate to contact another reviewers. It's all for me, Pighead Out.