By the power of truth, I, while living, have conquered the universe.
So ever since Cryptum was released the Universe has seen a hailstorm of threads wondering whether these two characters are one and the same. Looking at the evidence, I believe that it's very likely they are.
The Details:
The description we get of the Prisoner whilst he's inside the capsule is that he is roughly fifteen meters tall and eleven meters wide - pretty darn big. He had four arms, two legs and an insectoid looking head. A long, segmented "tail" attached to the base of the skull, tipped with a meter-long barb. Each hand had three fingers and a central opposable thumb.
The face of a Prisoner has been likened to that of a eurypterid. His head is flattened and has jaw mandibles.
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Comparison:
So in Halo Legends: Origins, we're treated to a short clip of the Gravemind present during the Forerunner-Flood war. He looks strikingly similar to the one we see in the trilogy (whether these are the same is another debate), but we can note a few similarities to the description in Cryptum.
The Gravemind, you will notice, has a set of jaw mandibles (similar to a Sangheili) and its head does indeed look slightly like an insect's.
Now there's an extensive list of eurypterids that existed in the Silurian era on Earth, I shan't begin to start naming them, however they all have a similar enough appearance to be used as a generalisation for this topic. Upon first examination, a eurypterid looks fairly similar to the Gravemind in terms of structure. But it's very difficult to get a proper look at the Gravemind as we've never actually seen the whole of his body, plus it has been taking on much more biomass from the other Flood forms which distort its form.
Looking beyond mere aesthetics, a truly defining piece of evidence I find is that Mendicant Bias conversed with the Gravemind for 43 years before forcing itself into Rampancy, defecting to the Flood and betraying the Forerunners. I find it to be far beyond coincidence that the test-firing of Installation 05 at Charum Hakkor - and thus the release of the Prisoner - occurred the same year.
So I've sort of got this one-sided argument going on here, it's up to you guys to either support this idea or bring it down with fire.
Discuss.