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I read this series around 1994. The protagonist was a boy who could summon elemental magic, or maybe it was just that he could summon elementals - I can't remember. A companion of the boy, an old man, was some kind of wizard or magic user who had a phoenix as a familiar. Another of the boys companions obtained a sword that could absorb the souls of its enemies. I think much of the series consists of the boy and his companions battling demons. I think the boys elemental magic makes the demons weak when they are in its presence.
At the climax of the series there is a battle between the old man (wizard) and a powerful demon made of diamond. The boy is also here but is unable to summon his elemetal magic because the substance the floor is made of prevents it. The old man is lying on the floor, almost dead, and scratches some symbols. The old man cries out "Uriel" and the angel Uriel is summoned. Uriel askes the demon why he has summoned him. The demon says, "I did not summon you, the old man did." The demon happens to be Uriel's most hated enemy, and Uriel is happy to finally find him alone, without backup. So Uriel and the demon battle and the demon is defeated.
Can anyone remember the series this is from?