Defiler

Companion: a summon, hireling or ally

ClassicHD

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Summoned by Summon Defiler

Its strength comes from the skill's level (required character level 18). Per skill level:

Skill level	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
Max Bound Souls	2	3	3	3	4	4	4	4	4	4	4	4	5	5	5	5	5	5	5	5
Damage Shared Between Bound Targets	26%	31%	34%	37%	40%	42%	43%	45%	46%	47%	48%	49%	50%	50%	51%	52%	52%	53%	53%	53%
Life	345	450	555	660	765	870	975	1080	1185	1290	1395	1500	1605	1710	1815	1920	2025	2130	2235	2340
Defense	125	145	165	185	205	225	245	265	285	305	325	345	365	385	405	425	445	465	485	505
Mana Cost	35	35.5	36	36.5	37	37.5	38	38.5	39	39.5	40	40.5	41	41.5	42	42.5	43	43.5	44	44.5

From the game files (monstats.txt, monlvl.txt). Picture: the original sprites from the game.