Humans have always been cooking up brand new ways to slice, dice, hack, and stab. You definitely wouldn’t want to tangle with any of the historical swords.
Deadliest sword or other *battle implement*, usually (not always) arrived at by exposure to the last such weapon and either improving it, or *engineering* a better one or tactic in response.
The best bladed weapons urumi is a sword with a flexible, whip-like blade, originating from the Indian subcontinent in Tamilakam, modern-day Kerala, Tamil Nadu and Sri Lanka. It is thought to have existed from as early as the Sangam period.
It is treated as a steel whip and therefore requires prior knowledge of that weapon as well as the sword. For this reason, the urumi is always taught last in Indian martial arts such as Kalaripayattu.
The word urumi is of Indian origin. In Kerala, it is more commonly called a chuttuval, from the Malayalam words for "coiling," or "spinning," and, "sword". Alternative Tamil names for the weapon are surul katti (curling sword) surul val (curling blade) and surul pattakatti.
It’s also disadvantageous to choose a weapon that takes much more practice to get good with than your usual weapon that’s one of the reasons spears were so popular: they were relatively easy to use, which means less time learning to use it, and more time actually improving your technique. Edge alignment seems particularly difficult to get right with a blade like that, which could mean years of practice just to get more than a scratch into your target.
Also, urumis are just as dangerous to the user as the target, and that’s not good in combat. Of course, it’s also possible that the idea simply didn’t occur to them.