Satan really has no power of his own. He only has what power God has given him.
Hollywood and New Agers don't have a clue about Satan's purpose for this present world. In some cases they try to show Satan is equal with our Heavenly Father (which is an old idea from Zoroastrianism).
In Isaiah 10, God uses the historical king of Assyria as a type for Satan.
Isa 10:5-6
5 O Assyrian, the rod of Mine anger, and the staff in their hand is Mine indignation.
6 I will send him against an hypocritical nation, and against the people of my wrath will I give him a charge, to take the spoil, and to take the prey, and to tread them down like the mire of the streets.
KJV
God is using the Assyrian (i.e., Satan) as His punishing rod. That is how we are to understand Satan's power God has given him to work.
This shows emphatically how God uses the title of the Assyrian as a type for Satan:
Isa 30:30-33
30 And the LORD shall cause His glorious voice to be heard, and shall shew the lighting down of His arm, with the indignation of His anger, and with the flame of a devouring fire, with scattering, and tempest, and hailstones.
31 For through the voice of the LORD shall the Assyrian be beaten down, which smote with a rod.
32 And in every place where the grounded staff shall pass, which the LORD shall lay upon him, it shall be with tabrets and harps: and in battles of shaking will he fight with it.
33 For Tophet is ordained of old; yea, for the king it is prepared; He hath made it deep and large: the pile thereof is fire and much wood; the breath of the LORD, like a stream of brimstone, doth kindle it.
KJV
Tophet is about the Valley of Hinnom, a valley southeast of Jerusalem where Judah in a fallen worship state sacrificed their children in the fire to the false god Moloch. And thus it became a perpetual burning garbage pit outside the walls of Jerusalem. Our Lord Jesus referred to it, geena in the Greek put for Hinnom. It is symbolic for the future "lake of fire" of Rev.20.
The main point to recognize in the above Scripture is that God (through Isaiah) is showing how "the Assyrian" is already judged and sentenced to that burning Tophet of the future. Thing is, God does not judge and sentence any flesh born man to the future "lake of fire" until the Great Day of His Judgment, which will only happen after Christ's future "thousand years" reign of Rev.20. By this we know our Heavenly Father is not talking about the historical flesh kings of Assyria here. He is talking about Satan who is... already judged and sentenced to perish in the future "lake of fire" event (Matthew 25:41). Thus here, in these above Isaiah 30 verses, God is pointing directly to Satan with "the Assyrian" title as a type.
In the previous Isaiah 10 example, remember God called the Assyrian "the rod of Mine anger". It means God is using Satan as a punishing rod upon even His Own people when they rebel against Him. We should see His purpose for Satan today as being no different upon those who rebel against God. Thus we are not to fear him.