When I was a kid back in the 70s the ad in all the comics was for "Crazy Crabs" ("land hermit crabs). I'd never seen such a thing and it was new. Begged Mom to get me one! I ordered two and figured since they must be tiny (like the sea ones on the beach) I'd get the large size.
In the mailbox a few weeks later came in a little box all cartoon-decorated. I opened up the box and two "monsters" were inside packed with a wet sponge and salad fixings - big, purple brutes in tiger-striped snail shells! WOW COOL thought I! Each one was a good three inches wide. I kept one and called "him" Rex, and the other I gave to my best friend, who named "him" Alvin.
Rex lived a couple years, as did Alvin. I fed Rex pretty much anything normal for people food - veggies, bits of meat, etc. The few times he pinched definitely hurt like heck! O lept him in a 10 gallon tank that had about 4 inches of potting soil in it and several wood branches. I made a house out of lego blocks. He spent a lot of time in the lego house as well as exploring his abode and climbing. The soil was well-packed and Rex loved to dig down into it and tunnel little burrows. After awhile he developed some unknown white-ishness on his legs, lost his large pincer, and two of his legs over time. Alvin didn't suffer so.
Very easy pet to care for, and pretty docile. bashful until they get used to being handled, but will crawl around in your hands, up your arm, etc. In the wild they do climb a lot. They only go to the sea once a year or so for breeding, and cannot live under water for more than an hour or so.
In the 90s I took my family to the Seattle Aquarium and they had new exhibit about coconut crabs! I found out many of these "land hermits" are really of this species. Given they grow big enough and are of this stock, you will definitely have a real monster of a crab.
Enjoy your crab! Make sure you find shells with openings somewhat larger than the current one. These guys do grow and will want to look for new houses on occasion. They actually measure new homes with their antennae, using them like a ruler to see if the size is right for moving into.