If you want to fill that crack, look into various epoxies. There is a clear epoxy that I've used a number of times for a thick, very shiny finish that would also fill that space and perhaps stabilize it as well.

I agree with the suggestion to make a side table. With pieces like that, I prefer metal legs to wood ones. Hairpin legs, for example, are super easy to find and install (but may also look dated in a few years).

It looks like it's about 4' in diameter - that's way too big for an end table in my house.

It looks like it would make a lovely table for 4 people. Once you finish it (in whatever way is appropriate - that's not my area of expertise), buy a piece of glass cut to the shape of the wood, so you can use the whole table and not worry about the crack.

That's interesting about the shrinkage. I just thought it was flawed or cut for some purpose. I was thinking end tables also, but it is too big for that unless you half them. I was also thinking a table to sit at but with the piece missing didn't know how I could make that work. I never thought of putting glass on it! Thanks so much for the suggestions and comments - love hearing them all!

What kind of walnut is that? Definitely not black walnut, is it English or Carpathian?

I'd make a whitewood dot, add black resin to the split with the dot in the split also..... ta-da! Pac-man.

That piece could crack even more before it is finished drying. Woodworkers typically leave large wood pieces to season for years before they are put to use. Once the wood has quit shrinking and moving, the one thing I can think of is to cut the piece all the way through and then use the resulting semicircles for a pair of end tables.

If you fill the gap (I wouldn't) use something very flexible, the gap will continue to expand and contract with seasonal humidity changes. Something that big could easily move 1/4-1/2" every year, even in a "stable" environment. .