It is not entirely clear whether he invented Sectumsempra or not. Severus's remark to Harry at the end of HBP seems to be claiming it as his, but then its appearance in his Potions text is minus all the working-out that accompanies Levicorpus, and one does wonder on what Lupin is basing his comment that "it was always a specialty of Snape's" (i.e., when would he have seen it). Some of this may be JKR mixing up who could know what, when. But overall it is not 100% sure.