They would like the ability to raise corpses as free-willed corporeal undead, but not have control over them afterwards. This could be useful as an indiscriminate fire-and-forget or attack area denial attack, just like a cloud of poison gas or minefield or similar, so it probably should cost something, but what?
AllyMany posters suggested using ally, but modifying it so that it conjures undead with no relationship to the caster. The Conjured enhancement (GURPS Dungeon Fantasy 9: Summoners p. 4) already requires that the summoned creature make a reaction roll for the caster. So this would seem to work pretty well (especially if the character has traits that give him positive modifiers for undead) but the character is then limited to the Allies that he's paid for, and can't conjure more without dismissing the current ones, and waiting 24 hours. The second feature of this approach is that the undead will take a variable time to arrive, based on the distance to suitable corpses and isn't limited by range or really by needing onscreen corpses. While this has advantages over having to, for example, go to a graveyard to do this, it puts the time for the undead to arrive in the GM's control and this might not fit the player's vision of how this ability works. Although it could be further modified with Trigger to make it work more like that.
You probably don't want to use this with the changes to the undead templates used for Aninmate Angry Dead (below) for obvious reasons, making the undead that are cojured likely to just stand around if you don't tell them what to do, which also doesn't really fit the concept.
Call the Damned
2 pointsYou can summon an undead minion (either a zombie or skeleton) by animating the closest corpse that isn't protected from such magic. When it arrives it rolls a reaction roll to determine if it will obey you (and may on an especially bad roll choose to attack you, see NPC Reactions p. B559). You can't use this ability again unless the creature is destroyed, or you dismiss it (at which point the corpse reverts to lifelessness); in either case you must wait 24 hours before summoning another one.
Statistics: Ally (Undead; 10% of character points; Constantly; Conjured +100%; Minion +50%; PM-10%) 
Call the Dammed, Mass
22 pointsAs above, but it works on entire graveyards and the like and summons (1dx5)+16 undead. Roll one reaction for the entire horde, but remember the modifier for relative numbers if it triggers a potential combat reaction!
Statistics: As above but add Ally Group 21-50. 
No Rest for The Wicked -Affliction (Advantage, Extra Life)This learned prayer from "Dungeon Saints" Pyramid #3/36 Dungeon Fantasy is a mostly cosmetic variant of the Raise Dead learned prayer (GURPS Powers: Divine Favor p. 13) that just specifies the target pays 25 points for the extra life by taking undead-themed traits. This probably isn't what Sombriero wants because it's pretty pricey at 58 points (as an independent ability rather than a learned prayer), doesn't inflict a specific undead template, and mostly keeps the personality of the deceased intact. It also is single target, and while you could add Area Effect to make a mass version that's only going to make it more expensive.
Affliction (Advantage, Alternate Form)This is probably the most straightforward way, just apply an undead template to a corpse! It still ends up pretty pricey though (possibly too pricey).
Animate The Angry Dead
51 pointsYou can animate a corpse turning into a skeleton, zombie or mummy (GURPS Magic p. 152) depending on its condition. Specify how long (up to a day) you want it to wait before rising and roll a contest of Will versus the corpses HT; you are at -1 per yard of range. These undead aren't under anyone's control but rather possessed by wrathful spirits; replace Slave Mentality [-40] and Reprogrammable [-10] with Bloodlust (Irresistible) [-25] and Obsession (Kill the Living; Irresistible) [-25].
Statistics: Affliction 1 (Advantage, Alternate Form +150%; Extended Duration, Permanent, +150%, Malediction 1 +100%; Only on corpses +0%; PM -10%; Variable Delay, minutes or hours +20%). Only on Corpses ends up as a +0% modifier because the ability to apply Alternate form that animates it (as opposed to just creating a dead zombie) seems to be mostly counterbalanced by it not working on the living.
Animate The Angry Dead, Mass
71 pointsAs above but animates every corpse in a half acre! In most likely casting conditions this will yield 128 undead!
Statistics: As above but add Area Effect, 16 yards +200%.
A Perk based on Infectious AttackThe logic behind Infectious Attack (p. B140) being a disadvantage seems to apply here, since the created undead aren't under your control. So maybe it's fair to allow this as a power perk (essentially controllable Infectious Attack) as part of a necromancy power. There isn't really any good way to make this into a mass effect ability though.
Touch of Undeath
You can take a Concentrate maneuver while touching a corpse and if you succeed at a contest of Will+Talent versus HT you can call a wrathful spirit to inhabit it. In 1d hours it will rise as a violent undead creature (as per Animate Angry Dead, above).
Innate AttackYou could treat this exactly like a cloud of poison gas, and just make it a modified innate attack. This has the disadvantage of being a bit silly, with the undead horde charging all over the landscape as an unbreakable unit.
28 pointsYou can create a horde of zombies that charge about the landscape at Move 3 attacking the closest living thing that they see, until it's dead at which point they will move to the next closest target. The horde takes up an area with a 4 yard radius and does 1d crushing damage per turn to anything in that area. It does not defend against attacks, has DR 0 and will disperse after taking 420 points of damage. You need at least 50 corpses within 100 yards to start this, and they will initially rally at any point within that 100 yards that you specify, at the time that you specify (within 24 hours).
Statistics: Crushing Innate Attack (1d; Area Effect 4 yards +100%; Extended Duration, Permanent, dispersed after taking 420 points of damage, +150%; Homing, vision +50%; Mobile, Move 3 +120%; Persistent +40%; PM -10%, Trigger, At least 50 corpses -20%; Variable Delay, minutes to hours +20%)