Simon Legree is the embodiment of pure evil. He is a slaveowner who is not bothered with the moral implications and ethical consequences of his actions. He is one who enjoys tormenting his slaves and takes pride in exercising complete control over them. Simon Legree occupies the role of the ultimate force of negation which exists in slavery. Stowe needed to develop the vision of a master that embodies the true horror of slavery. The sadistic joy that Simon takes in the hurt and torture of his slaves makes him such an important character in the narrative.
For Stowe, the intent of showing slavery as the ultimate form of evil only happens with Simon Legree. Through Simon's characterization, the reader understands how bad slavery is and how much a moral affront it truly is. It is in this condition in which Simon operates. In Simon's desire to be seen as ultimate master, Stowe has offered a vision as to how morally corrupt slavery is. It is in this light in which Simon operates in the narrative.
Simon Legree is a brutal man who purchases Uncle Tom and Emmeline for his Louisiana plantation along the Red River. His plantation holds the worst examples of cruelty to slaves; they are starved, overworked, and often beaten. He believes he is the highest master for the slaves, and therefore doesn't let his slaves acknowledge God as a master. When Tom holds to his Christian values and doesn't accept Legree's offer to become a slave driver, he gets on Legree's bad side. Simon Legree keeps a slave mistress named Cassy. Eventually, Cassy finds the courage to escape and runs off with Emmeline, enraging Legree. He asks Tom about where they went, and when Tom refuses to answer him, Legree has Sambo and Quimbo, his cruel slave drivers, beat him to death.
While these are important points to keep in mind when describing Legree, there are some more details about him, the characters with whom he interacts, and the over all plot of Uncle Tom's Cabin in the study guide here on eNotes. I have provided the link below.