Cleave and Great Cleave, poor feats though they are, are roughly similar to sweeping strikes or mighty/massive swing. Except, you know, that they trigger very rarely instead of “every attack.”
Whirlwind Attack is also a kind of “melee area attack,” but is, again, a very-poor feat, which in this case requires four more very-poor feats.
Savage Species actually offers a feat called Area Attack, which, along with Crush and Stamp in the same book, allow Huge-or-larger creatures various types of area-affecting attacks. All of these are non-weapon attacks, and only attack once as a full-round action, so they are bad feats, and then Huge size is rarely if ever available to players and certainly doesn’t seem very ninja-y.
Tome of Battle generally offers more options, particularly in the Iron Heart discipline, but in E6 the only one of those maneuvers that’s available is steel wind, which attacks two opponents as a standard action. Not terrible, but not particularly exciting either.
There is also the Snap Kick feat in Tome of Battle, which allows you to make an extra unarmed strike whenever you would make one or more attacks. You can use this against the same target or a different one.
Sword and Fist has the Circle Kick feat, which works like Snap Kick does except that it only applies during full-attacks. If you had both Circle Kick and Snap Kick, you would get two extra unarmed strike attacks in a full-attack.
While we’re mentioning extra unarmed strikes, it’d be silly to ignore flurry of blows. Again, full-attack only.
And the whirling frenzy rage variant is yet-another source of extra attacks in a full-attack.
That appears to be it, which is pretty slim pickings. Steel wind is solid enough, though nothing truly special, and Snap Kick is a solid feat if not exactly what you want. Most of the rest (and all of them worth taking) are just ways of getting more attacks, which can be used on separate opponents but are better used to focus down the first one. None of them allow you to turn every attack into an attack on multiple targets, the way sweeping strikes or mighty/massive swing can.
A thought does occur to me, though: Paimon, the Dancer is a vestige from Tome of Magic, that a 3rd-level binder with Improved Binding can make pacts with. Such a pact offers both Weapon Finesse and Whirlwind Attack, as well as the very-interesting dance of death feature, which is basically a whirlwind attack that gets to move. Three levels and a feat gets you quite a bit that way, plus options for switching to a different vestige if warranted (Naberius offers excellent social skills and ability-damage healing, Malphas offers fantastic scouting and spying, etc.).
And if you are spreading out your attacks that much, they need to be worth something. Poison is a way to amp up the damage of your attacks, but tends to be usable only once. There is a way around that, though: the Poison Spell feat from Drow of the Underdark applied to a multi-hit spell like chill touch would apply the same poison over and over. And a 3rd-level duskblade (Player’s Handbook II) can channel chill touch into a weapon attack.
If you then toss this onto a reach weapon of some kind, you can reach an enormous area of the battlefield with your poisoned attacks. This does lose Paimon’s Weapon Finesse—that only applies to short swords or rapiers, annoyingly—but E6 does get bonus feats, or you can try the Dungeon Master’s Guide II feycraft weapon template. And, conveniently, the best weapons for this are very “ninja-y” weapons, mostly exotic weapons that either originated in Asia in the real world, or are loosely based on Asian weapons. In particular, your best options are
- rope dart or meteor hammer (both Dragon vol. 319)—two-handed, enormous 15-ft. continuous reach, that is, it does not leave gaps at 5 ft. or 10 ft., and it can be finessed. The only difference between these weapons is damage type (piercing vs. bludgeoning).
- whip-dagger (Dragon vol. 353 or Dungeon vol. 134)—one-handed, and again, enormous 15-ft. continuous reach, but no attacks of opportunity.
- kusari-gama (Dungeon Master’s Guide)—light, 10-ft. continuous reach.
- spiked chain (Player’s Handbook)—two-handed, 10-ft. continuous reach, finesse-able.
So a 3rd-level duskblade/3rd-level binder (or, perhaps better, a 3rd-level duskblade/2nd-level binder/1st-level knight of the sacred seal) could cast chill touch with Poison Spell, and then potentially poison everyone he can reach. Note that you’ll only get at most one attack the same turn as casting chill touch, so it behooves you to be stealthy and decide when the fight begins. Seems mystical, sneaky, a fair bit on the underhanded side what with all the poison: rather ninja-like, really.
For more tips on using poisons well, see the Arsenic & Old Lace Handbook.
For actually sneaking with these classes, which don’t get Hide or Move Silently as class skills, I recommend Martial Study from Tome of Battle to get Shadow Hand maneuvers. In addition to making Hide always a class skill for you, these options can be very good, such as cloak of deception for turning invisible during your turn (to be safer from attacks of opportunity while you perform your dance of death) or shadow jaunt (standard-action 50-ft. teleport, requires line-of-sight). This also enables you to take Martial Stance, to get child of shadow or island of blades (thanks to the dance, child of shadow should be quite reliable), which then makes the Shadow Blade feat, also Tome of Battle, available—and you want that, since it adds your Dexterity bonus to damage.
Getting Move Silently in-class is obnoxiously difficult, however. Flexible Mind from Dragon vol. 326 can do it if that is available. If not, that leaves only the Aereni Focus feat from Player’s Guide to Eberron—which is elf-only. Of course, you like Dexterity quite a bit, so being an elf is not so bad. The Constitution penalty is quite painful though. Unearthed Arcana offers the arctic elf has −2 Strength instead of Constitution, and furthermore gets a +2 racial bonus on any one Craft skill of your choice (choose poisonmaking).
There is another option that allows you to skip duskblade entirely: a tentacle whip symbiont from Eberron Campaign Setting. This has 15-ft. continuous reach, a built-in poison, and can deliver spells without even needing arcane channeling (and then some). You would still need to get chill touch or similar from somewhere, but you would have more options for that, plus you could dip other useful things like psion for minor psionic creation or cleric with an eye towards Lolth’s Caress. Daelkyr half-bloods can get one of these at 10th level, as can 5th-level impure princes, but neither of those are possible in E6, so you’re left with Magic of Eberron’s suggestion that one is worth approximately 8,000 gp. Good luck finding a seller, though.