Like the Templars, we have no reason to think the Hashashin survived their destruction by the Mongols in 1275. But, also like the Templars, their legend and tradition have come down through the years. For example, the Hashashin called their fighters Fedayeen, much like Islamic Jihadists call themselves today. One might also make a case that today's suicide bombers are similar to the Hashashin in the expectation that they will die accomplishing their mission. The Hashashin often employed public attacks on their targets, where the Hashashin had no hope of escape.
The Ismaili Shiite Muslims do exist today, but the Hashashin were always a small subset of that group.
This is where The Templar Concordat bends history a bit by keeping the Hashashin alive and well into the present day.