Energon crystals are a more powerful form of Energon when compared to cubes. However, they are extremely unstable, emitting radiation which can be fatal to Transformers with prolonged exposure. It also risks detonation with temperature increases.
Megatron led the Predacons to Earth with the hope of finding Energon, but the crystals were so powerful, the radiation from them threatened to extinguish the sparks of the Predacons and Maximals by triggering a catastrophic energy overload after more than a few minutes of exposure. To avoid death, both factions took beast modes to allow them to function outside their respective vessels.
Terrorsaur later found a mountain filled with extremely unstable Energon crystals, causing it to float in the air. Although the mountain was destroyed by Optimus Primal and Rattrap, it did empower Terrorsaur enough to defeat Megatron...for a few nano-kliks.
When the Vok, who seeded the Energon on the planet, discovered the presence of the Maximals and Predacons, they attempted to destroy the planet by detonating the Energon crystals with the Planet Buster. Optimus Primal managed to destroy the weapon with a stasis pod reconfigured by Tarantulas to serve as an escape pod, but the initial heatwave and subsequent Transwarp explosion caused the Energon crystals in the area surrounding the Axalon and Darksyde to detonate or convert to Energon cubes .
This allowed the Maximals and Predacons to stay in humanoid mode outside their bases for prolonged amounts of time without the effects of Energon buildup, although many of them were still known to use their beast forms for travel purposes(As well as when exploring some areas where the Energon was still raw.