The Forerunners would not have wanted to honor Mendicant in any way, so why would they build a tomb or write an epitaph for him? Instead, I think that Epitaph could be a monument for Offensive Bias who they created to combat Mendicant Bias and Gravemind. They constructed the building that Epitaph is at the top of to honor/store Offensive Bias which would have been seen as a savior of sorts. This may have already been thought of, but all I see is "Oh, Epitaph is Mendicant Bias's tomb! Let's go find the tomb everyone!" so I thought this would be fresh.
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Well, why would they made safe havens from the rings if they weren't planning on using them? I don't believe that all the Forerunners died. I believe that after the rings fired they returned the sentient species to their repesctive worlds and named Humans (for what ever reason) their heirs. Then they disappears to parts unknown to see how it all developed without their influence, or maybe they reached the "Transcendent" level and traveled the universe to hunt down the Flood and attempt to stop it from spreading from where ever it came from. Attack it at the source, you know? I don't think that they're dead though.