Dieter did a good job of explaining פַּחַד pachad and the LXX equivalent. I won’t repeat what he said instead I want to take a different angle. Namely this fear has a spiritual source and both Elohim and the gods of the nations are capable of generating this fear, panic, dread, sense of wrath/anger (of the gods).
“Then they attacked all the cities in the vicinity of G’rar, for a PANIC from Adonai had overcome them. They plundered all the cities, for they were full of spoil.”
Divrei-HamYamim Bet (2 Ch) 14:13 CJB
But notice it can also come from unclean spirits
“Then he took his firstborn son, who was to have succeeded him as king, and offered him as a burnt offering on the wall. Following this, such great ANGER came upon Isra’el that they left him and went back to their own land.”
M'lakhim Bet (2 Ki) 3:27 CJB
So essentially this fear of the Lord or this fear from a fallen elohim is a spirit of sorts. Similar to the example in the divine council that Micaiah describes.
“Adonai asked, ‘Who will entice Ach’av to go up to his death at Ramot-Gil‘ad?’ One of them said, ‘Do it this way,’ and another, ‘Do it that way.’ Then a SPIRIT stepped up, stood in front of Adonai and said, ‘I will entice him.’ Adonai asked, ‘How?’ and he answered, ‘I will go and be a deceiving SPIRIT in the mouths of all his prophets.’ Adonai said, ‘You will succeed in enticing him. Go, and do it.’ So now Adonai has put a deceiving spirit in the mouths of all these prophets of yours; meanwhile, Adonai has ordained disaster for you.””
M'lakhim Alef (1 Ki) 22:20-23 CJB
It’s a spirit in this context for the passage you reference that simply overwhelmed the enemies that they panicked with fear. Notice how it’s phrased, “Adonai put a spirit...” it’s His doing, in order words, just because a spirit of fear came over the Ethiopians it was the fear of Adonai that caused them to panic not some random fear spirit, it belonged to Adonai, or under His jurisdiction.
The Ethiopians may have been more powerful or numerous v9 of your passage but the fear of Adonai through the spirit of fear crippled them.
This is why there was so much cursing prior to warfare in ancients times, (today it has a new name, psychological warfare), the enemies were invoking a spiritual dualism in the spiritual realm to win by overwhelming the enemy through a spirit and depending whose G-d/god or fallen elohim or daimonion was stronger winning in the natural was practically guaranteed. David and Goliath do a similar thing.
Today we simply call it psychology but at the root is the psyche which is not at the level of the body or the soul though it influences the mind, emotions and will, affecting the body but it’s at the spirit level.
The fear of Adonai so overwhelmed them in their psyche that they panicked and shuddered with fear. It was facilitated by a spirit other than the human spirit/psyche, it paralyzed their soul (thinking fixated, emotions overwhelmed, will crippled) that their bodies consequently were affected negatively and lost though they were an army of a million warriors
Psychology is manipulating the soul and body though the spirit or external spirits. Very powerful but Adonai is superior and Other, so join the winning side if you’ve not already.