NPCs composing that spawngroup :

an_alarm (10%)

an_explosion (10%)

an_inactive_trap (10%)

a_blast_of_heat (10%)

a_dark_entity (10%)

a_formless_terror (10%)

a_magi_construct (10%)

a_null_field (10%)

a_wardstone (10%)

a_wizard_construct (10%)

NPCs spawning around that spawngroup :
(Max range : 200)

necropolis_21, range=145 (1299,673,-80)

an_amorphous_mass (100%)

necropolis_44, range=97 (1465,664,-155)

an_amorphous_mass (50%)
an_amorphous_mass (50%)

necropolis_45, range=86 (1457,656,-155)

an_amorphous_mass (50%)
an_amorphous_mass (50%)

necropolis_92, range=49 (1363,591,-155)

an_amorphous_mass (100%)

necropolis_158, range=144 (1307,681,-81)

an_amorphous_mass (100%)

an_out_of_phase_spid_190074, range=145 (1327,464,7)

an_out_of_phase_spider (50%)
a_phased_out_spider (50%)

an_inactive_trap_190117, range=124 (1288,572,-153)

an_inactive_trap_190119, range=192 (1366,769,-82)

an_inactive_trap_190124, range=98 (1471,661,-153)