Descendant is both an adjective (meaning either moving downward or descending from an ancestor) and a noun (for someone who descends from someone else). Descendent is a less common variant used indiscriminately in place of descendant in all its uses.
There is a traditional distinction between the two forms, and some English reference books still give it credence. It is that descendent is the adjective and descendant the noun, the logical basis for this being that the older adjective should follow the Latin root and the derivative noun should follow the later French form. But while some very careful writers still honor this distinction, it is not borne out in broader usage. For the most part, the two words are used interchangeably, and descendant is the preferred spelling.