When your only Archfiend monster is an Equip Spell Card equipped to “Relinquished”, you cannot Normal Summon or Flip Summon “Terrorking Archfiend”.
While “Skill Drain” is active, you still need to have an Archfiend Monster Card on your side of the field in order to Normal Summon or Flip Summon “Terrorking Archfiend”.
You can Set or Special Summon a “Terrorking Archfiend” even if there are no Archfiend Monster Cards on your side of the field.