Standard disclaimer: I am not a geologist. I am not a geographer. I am not most things. Take all of my discussions with a pound of salt.

Your crater is roughly 600 miles in diameter, making it 6 times larger than the buried Chicxulub crater in the Yucatan, meaning it would have released roughly 200 times the energy (i vageuly recall think energy in this situation required to produce a creater of a particular size is the is proportional to size cubed - i'd have to go do some research to be sure). If the estimates that nothing much larger than a rat survived the Chicxulub impact, then a similar mechanism to produce a crater 6 times the diameter size would not be good for intelligent life or anything resembling cities. A crater that's roughly 10 percent of the planetary radius may not be compatible with a stable crust for a long time afterward.

As tilt has suggested, however, if a different mechanism (or different type of energy release) were to occur, then the results could be radically different. If the blast was more planar than spherical, then the energy release would be much smaller. It would still be massively destructive. Likely there would be a huge atmospheric shock wave sufficient to knock down stone buildings around the world. There would be severe ground shaking for a long way out, too. Assuming a coastal location, figure that the water would effectively be pushed back into a tidal wave several kilometers high. This wave would likely go around the world several times. Landfall of the wave would run several hundred kilometers inland, pushing everything inland and then scouring the coastal plain to bedrock on the way back out. I wouldn't expect to really find much evidence of life after that sort of event, either.

I can't think of a scenario for an explosion that would leave a 600 mile diameter crater and still have a typical fantasy biosphere afterwards. You might have something like a plain of glass left at the detonation site, but it would still be fairly smallish.

One option is to invoke something like a wormhole where most of the blast would end up going away in the sense of in a direction different than the usual 3 dimensions. The 600 mile section would effectively cease being there with a little bit of energy backlash for pretty colored lights. The implosion of atmosphere and ocean into a hole that large would be quite messy, but probably still survivable for places moderately far away. If the destruction was mostly planar there wouldn't be the problem of a huge hole through into the molten mantle or nasty mantle reflow effects. Just the sort of nasty that you're looking for because it's easily tunable. The fun part about this kind of scenario is that some parts of the "missing" landscape might still exist "elsewhere" and there might be weakened portal-type areas within the blast area that allow access.

If you have a low peninsula then a tidal wave of sufficient height would sweep over it, doing all manner of damage (look up channeled scablands for an example of the sorts of things large volumes of water can do in a short time). If the peninsula is mostly sediments, then channel formation is a distinct possibility. A couple of small ranges of hills flanking the desired channel area could well direct enough water energy to assure channel formation.