In short, the Sartan of Abarrach had degenerated in magic much more significantly than they believed.
In long: Sartan (and Patryn) magic works by assessing the possibilities and probabilities, then using the appropriate rune structure to call up the desired version of reality.
The more improbable something is, the more strength was required to pull that into existence.
There's multiple references throughout the books of Haplo and Alfred (and, on rare occasion, others) assessing the possibilities.
My favorite version of this being described is actually from Alfred in the final book, The Seventh Gate , while the group is under attack and he's trying to figure out an escape.
Note how even Alfred, the Serpent Mage, doesn't immediately come up with a good solution: Possibilities whirled through Alfred's mind, dazzling, confusing, terrifying.
I'll make the cavern roof collapse.
(No, that would kill us all!)
I'll bring a fire dragon up through the floor.
(No, same outcome!)
A flower garden will suddenly appear out of nowhere.
(What good will that do?!)
The lazar will attack.
(Someone might get hurt . . . ) The floor will open and swallow me up . . .
(Yes!
That's it!)
(Additionally, he is later berated for choosing that last possibility, because there was a chance he would have dropped them into a magma pool.)
Throughout Fire Sea , the Necromancers have occasionally given us history of the art in their world.
According to them, necromancy was a lost art among the ancient Sartan (Alfred's people), and they had rediscovered it.
Through trial and error, they perfected it, better preserving the corpses, discovering the 3-day rule, and so on - remember the comments early in the book about the "old dead" being in bad repair?
So, The Necromancers of Abarrach, despite being Sartan, no longer see/sort through the possibilities in the same way a "full" Sartan like Alfred can.
They never rediscovered the correct way to use the resurrection runes.
(Identical rune structures can be used in different ways, producing different - or incorrect - results.
See the appendix in Dragon Wing , titled "Magic in the Sundered Realms/Excerpt from a Sartan's Musings" for more information)
They incorrectly interpreted what the shadow/spirit attached to each corpse was, and what was supposed to happen to it after death of the body.
Their strength was sapped due to keeping themselves alive in the toxic fumes that permeated the atmosphere, so it was unlikely (but possible) they could have performed a true resurrection like Alfred, even if they knew the correct way to use those runes.
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It's been a long time since I read the books, but as I remember, the Sartan of Abarrach had forgotten/lost the true strength of Sartan magic.
Alfred, however, has not degenerated so.
In fact, as the serpent mage (even though he may not be consciously in control of his full power, IIRC
he resurrected Hugh without conscious action), he may be uniquely gifted to resurrect without the same ill-effects the lazars suffered.
Keep in mind Hugh was not unchanged by his return, although he did appear to be much better off than, for example, Jera.
Given Hugh's curse when he returned, it's possible Alfred is just able to tap into a completely different type of resurrection than that the Sartan (of Abarrach, at least) used.
Alfred notes that the magic of the Sartan of Abarrach is all about death -- perhaps their resurrection techniques, focused on death, result in zombies and lazars, while what Alfred did is focused on life, which causes Hugh's return to life but also his curse.
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I always understood it as a reapplication of possibilities, the rune construct used on Hugh the Hand for example called forth the possibility that he didn't die.
That's why when he was killed he summarily returned to life, this would be different than a rune structure that allowed for the possibility that the spirit couldn't leave the body.
But that's just my understanding.
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It was stated near the end that it was the reasoning behind Alfred's use of necromancy that didn't turn Hugh into a lazar.
The Abarrach Sartan used necromancy for selfish purposes.
Alfred used it out of compassion and love.
That was the difference described in the books.
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Another possibility is that the Lazars were resurrected Sartans.
They had magic of their own, so when they felt tormented by being brought back to life they instinctively tried to break the spell which causes them to be stuck between life and death.
Hugh, being a regular human, didn't have the kind of power that was required to fight Alfred's spell, so he stayed alive.
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In The Death Gate Cycle; why didn't Hugh the Hand turn into a lazar when Alfred got him back from the dead?
Why didn't Hugh the Hand turn into a lazar?
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Hugh, being a regular human, didn't have the kind of power that was required to fight Alfred's spell, so he stayed alive. Alfred notes that the magic of the Sartan of Abarrach is all about death -- perhaps their resurrection techniques, focused on death, result in zombies and lazars, while what Alfred did is focused on life, which causes Hugh's return to life but also his curse. The fact that the Spartan of Abarrach lost their strength of magic combined with the reasoning behind Alfred’s use of necromancy meant Hugh didn’t turn into a lazar. Furthermore, Hugh’s human powers kept him alive.
The reason for this is that the Spartan of Abarrach lost much of their powers of magic. The reasoning behind Alfred’s use of necromancy meant Hugh didn’t turn into a lazar. Hugh’s human powers kept him alive.