A few characters in Bleach use mad science and boundary-pushing kido spells to try and create brand-new possibilities, and the results are often devastating. The best examples of this are Kisuke Urahara, the scientist who invented the Hogyoku, and Sosuke Aizen, who stole it to create an Arrancar army. But for the 8th Espada, Szayelaporro Grantz, not even that was sufficient. He wasn't perfect yet.
A recurring idea in Bleach is the futile existence of a Hollow, because for all their twisted and beastly power, Hollows can never sate their hunger or escape their fate, even if they become a Vasto Lorde. But Szayelaporro Grantz thought up an angle, and he pursued it with all his might.
Only a few Hollows of the Vasto Lorde class exist, because it takes so many Hollow combined to create one, and such beings have enormous appetites. The few who exist represent the maximum extent to which Hollows can gain power, in their futile quest to escape their twisted nature. For example, Barragan Luisenbarn was a Vasto Lorde who ruled Hueco Mundo, but even that turned out to be an empty existence. But another Vasto Lorde by the name of Szayelaporro Grantz had a different idea: what if he used the power of science to set a clear goal to escape the Hollow curse? With a methodical approach, he aimed to become truly perfect, something that not even Barragan, Ulquiorra or Harribel had managed. He was a visionary, or at least, he felt like one.
So, Szayelaporro split himself in half to reset himself to the Adjuchas stage and attempt to modify his body and evolve into the perfect being. His other half became his "brother," Ilfort Grantz, and Szayelaporro used his extensive knowledge of spiritual energy and Hollow anatomy to set up a lab and begin his research and experiments. He amassed a whole platoon of Fracciónes and experimented on them, and used surveillance bugs to spy on his enemies to study them in detail -- he later even did this to Uryu, who fought Ilfort. Szayelaporro carefully modified his released form, La Lujuriosa, and gave it extra abilities, ranging from the creation of voodoo dolls to cripple his enemies, to a rebirth function called Gabriel. Szayelaporro used these powers and more to beat Renji and Uryu Ishida into submission, but then he met his match: Captain Mayuri Kurotsuchi.
Captain Kurotsuchi was also a scientist, and what ensued was a scramble to control each other's powers with mad science and manipulation. Like Szayelaporro, Mayuri liked to use spy bugs, and with their use, he figured out a way to neutralize Szayelaporro's abilities, and get the upper hand. Szayelaporro even used the Gabriel ability to re-create himself, like a phoenix, but it was for naught. Mayuri finished him off with an experimental drug that accelerated his mind and left him paralyzed, then stabbed him in the heart to conclude the battle.
What exactly happened? The problem was that Szayelaporro sought perfection through science, and as Mayuri explained, that was a contradiction. Scientists face the paradox of needing to improve forever but never reach perfection, because perfection represents a dead end. Nothing can change or grow, and it might even represent despair. Szayelaporro, in his arrogance, failed to realize this, and it made him complacent about his powers. He had failed to respect the true nature of science, and he now had centuries to contemplate that, trapped in his own mind, before his physical body finally succumbed to Mayuri's zanpakuto strike.