I'd like a PC in my campaign to slay his foes by using a melee or ranged weapon like a sword or spear then have those foes rise immediately to join him in the fight as zombies or some other kind of undead creatures.
My initial thought was to use the metamagic feat Fell Animate that, in part, says, "You can alter a spell that deals damage to foes. Any living creature that could normally be raised as a zombie and that does not possess more than double your Hit Dice, when slain outright by a fell animated spell, rises as a zombie under your control at the beginning of your next action" (Libris Mortis 26–7).
I'd apply the benefit of the feat Fell Animate to, for example, the 2nd-level Drd spell flame blade (Player's Handbook 231) or the 3rd-level Moon domain spell moon blade [evoc] (Spell Compendium 143) or maybe even—with my permission (I'm the DM)—the 4th-level Sor/Wiz spell Darternae’s sword [evoc] (Celebrity Game Table column "The Praemal Campaign") and make the magically-created weapon last all day via the feat Persistent Spell (Complete Arcane 81) and the prestige class spelldancer (Magic of Faerûn 37–8).
Will this plan work? Is there a better one? That is, is there a plan that's available at lower levels, that's more convenient, or that's less resource-intensive that allows killing a foe with a sword (or, if need be, an arrow) that thereupon animates that foe as an undead creature immediately?
Extreme cheese—Pun-pun, complicated persistent time stop shenanigans, etc.—will be frowned upon but not dismissed outright if entertaining. Further, even third-party material is acceptable to accomplish this task, but the more official the material used to do so, the better. Also, although not strictly necessary, try to keep in mind that the PCs are level 15, that the campaign's binds limit PCs to taking every other level in the same tier 1 or 2 full-casting class (e.g. a level 15 PC could be a Clr7/Wiz8 but not a Wiz15), and that the new PC will be entering play with NPC wealth of only 59,000 gp.