Not upon the distance to or level of nearby undead. Furthermore, the zone modifier for this item appears to be at least somewhat arbitrary and based upon how "undead" oriented the zone is. I based the concludion in the previous sentence on the stats I got in various zones.
Plane of Knowledge, Guild lobby, the bazaar,the Void and Upper Guk all gave what appear to be the minimum stats:
AC 21, HP 231, Mana/End 237, Sta 18+1, Str/Int/Wis/Agi/Dex/Cha 10, Mag 18, Fire 6, Cold 12, Dis 9, Pois 12, Dot Shield 1, Stun Res 1, Heal Amt 1, Spell Dmg 1.
Estate of Unrest gave the same stats in both the Greater Unrest mode and as a low level zone:
AC 41, HP 442, Mana/End 455, Str/Int/Wis/Agi/Cha 20+1, Dex 20, Sta 34+2, Mag 34, Fire 11, Cold 23, Dis 17, Pois 23, Dot Shield 3, Stun Res 3, Heal Amt 2, Spell Dmg 2, Clairvoyance 1.
Frostcrypt gave the exact same stats as Unrest. These appear to be the true maximum, though I wouldn't be surprised if there is room for a zone that gives exactly double the ac/hp/mana/end and stats/resists as the minimum.
Lower Guk gave:
AC 37, HP 396, Mana/End 407, Str/Int/Wis/Agi/Cha 18+1, Dex 18, Sta 30+2, Mag 30, Fire 10, Cold 20, Dis 15, Pois 20, Dot Shield 3, Stun Res 3, Heal Amt 2, Spell Dmg 2, Clairvoyance 1.
Hills of Shade gave:
AC 34, HP 373, Mana/End 384, Str/Int/Wis/Agi/Dex/Cha 17, Sta 29+1, Mag 29, Fire 9, Cold 19, Dis 14, Pois 19, Dot Shield 2, Stun Res 2, Heal Amt 1, Spell Dmg 1.
Dagnor's Cauldron is the odd one, and the one that made me say the values are somewhat arbitrary. There are a lot of undead here, but aqua goblins alone outnumber them. Despite that the charm gives the stats:
AC 39 HP 419 Mana/End 431 Str/Int/Wis/Agi/Cha 19+1, Dex 19, Sta 32+2, Mag 32, Fire 10, Cold 21, Dis 16, Pois 21, Dot Shield 3, Stun Res 3, Heal Amt 2, Spell Dmg 2, Clairvoyance 1.
I didn't record complete stats for the other zones I passed through to the ones listed above but all the ones I looked at, including Field of Scales in SoD (which has a significant undead population, though thoroughly outnumbered by living mobs), gave under 30 ac and under 300 Hp and similar reductions in other stats. However, if they had any undead mobs at all in them, like Greater Faydark where they are limited to a relative handful of "a decaying skeleton", they gave at least a 10% bonus over the minimum values listed above.
edited to add a couple zones and get complete stats for a couple zones I only noted ac/hp for originally.
Edited, Sep 14th 2010 6:25pm by Taadieri
Edited, Sep 14th 2010 6:29pm by Taadieri
Edited, Sep 14th 2010 6:29pm by Taadieri