NPCs composing that spawngroup :

an_alarm (10%)

an_immobilizer (10%)

an_inactive_trap (10%)

a_blast_of_heat (10%)

a_dark_entity (10%)

a_low_hum (10%)

a_magi_construct (10%)

a_null_field (10%)

a_wardstone (10%)

a_wizard_construct (10%)

NPCs spawning around that spawngroup :
(Max range : 100)

necropolis_42, range=67 (1695,3,-155)

an_amorphous_mass (50%)
an_amorphous_mass (50%)

necropolis_92, range=43 (1708,96,-152)

necropolis_a_blast_of_heat148, range=0 (1732,59,-157)

a_blast_of_heat (100%)