Oh man, I can tell you about it from experience. I took 20 seeds once and the vasoconstriction made all of my limbs burn. I felt as if my heart was beating ridiculously slow, my hands and feet were tingling, it was as if I was constantly on the verge of fainting. There were great surges of euphoria and that was the only trip that I ever got a genuine kaleidoscope on. I completely lost my ability to rationalize, I had the memory of a goldfish and I was convinced I was dying. I went down to the basement to listen to music and suddenly, a light ray in the corner of the room started spinning in a circle around the room incredibly quickly, which gave me the impression I was in a dome. Then I saw these gray entities that looked like they were made of slime or clay. They had sunken in eye sockets and they rose out of the ground in a strangely gloopy way and started walking towards me. I ran upstairs into the light and I was fine. But that trip made me so incredibly confused in such a strange way... I was baffled that any human being could have the faith to participate in life, not knowing if any of this was even real. I felt that it took so much faith that existence is real just to simply comply with the rules of life, that I didn't understand how anyone survived in today's world.
You cannot fatally overdose on HBWR.
Here's a good trip report of a HBWR overdose :