It's very much like the live-action movies' Allspark - a bigass cubelike thing that's flying through space and full of knowledge and power. Except it was from the G1 comics twenty years before the films.
Interestingly, Underbase energy affected Transformers in similar way to Beast Wars energon. That is, Transformers with organic shells or organic components were protected, while 'normal' geewun Transformers tended to be killed by it. A lot. (It killed Jetfire, Bluestreak, Brawn, Gears, Goldbug, Mirage, Hoist, Hound, Jazz, Blaster, Nautilator, Seawing, Snaptrap, Skalor, Overbite, Tentakil, Chase, Goldbug, Freeway, Searchlight, Rollbar, Wideload, Slag, Swoop, Snarl, Sludge, Divebomb, Razorclaw, Tantrum, Rampage, Headstrong, Grimlock, Laserbeak, Buzzsaw, Octane, Silverbolt, Skydive, Slingshot, Air Raid, Fireflight, Astrotrain, Blitzwing, Prowl, Strafe, Scattershot, Lightspeed, Nosecone, Afterburner, Hun-Grrr, Rippersnapper, Blot, Cutthroat, Sinnertwin, Omega Supreme, Ratbat, Thundercracker and Skywarp, but left the 'organic' Transformers alive.)