Make the invoke effect of the Tsurugi of Muramasa enhance it to give double damage for a limited time, but also become bloodthirsty during that time. For an immediate effect, it could also cause fear when invoked.

The Tsurugi of Muramasa should act bloodthirsty like Stormbringer does, because historically all swords made by Muramasa were reputed to be bloodthirsty or traitorous.

Bisecting a monster with the Tsurugi of Muramasa should have a chance of scaring nearby hostile monsters into fleeing.

Mithril body armor prevents the Tsurugi of Muaramasa from bisecting you (or a monster wearing it, for that matter).

As a potential Tsurugi of Muramasa buff, extend its bisection to Large monsters. Huge and Gigantic monsters still can’t be bisected, but Large ones (a fairly numerous set of monsters) become vulnerable to its main special attack.

Invoking the Tsurugi of Muramasa causes a fear effect similar to reading scare monsters and gives a temporary effect equivalent to increase damage (possibly only on hits made with the Tsurugi, possibly with any weapon).

It was also pointed out that triggering a temporary effect by invoking could get tedious. Another suggestion was to give it some sort of passive fear-causing effect when wielded (in addition to the one-time scare when invoking it).

The Tsurugi of Muramasa should allow Samurai to use additional “techniques” besides just bisection, with more becoming available with greater two-handed sword skill.

Raise the instakill ratio of Vorpal Blade and the Tsurugi of Muramasa, but only make it work if the target has under 50% of its hit points. Thus, the player can’t be instakilled by it if they stay above that HP threshold.

If you get bisected by the Tsurugi while polymorphed into a long worm, you get cloned instead, with a tame copy of you splitting off.

The Tsurugi of Muramasa always bisects long worms, cutting them in half (unless they have only a head segment).

Vorpal and bisecting weapons don’t get dull when you engrave with them.