Nice! It's good to see some Avewnus background, especially with Agrach and Vis involved.
Edit: Now, time for mass nitpickery:
"Just another day" , thought Xarph, when he was flying to raid. He commanded a small stack of 12 Hellflame Dwagons, each mounted by his personal Bezekiwa knights. The flight darkened the skies.
No...not darkened. The dwagons were blazing as lava, leaving trails of flame with their wings, mounted by the Bezekiwa knights in black armor, commanded by Chief Warlord of Avewnus, Xarph himself. Their target was Offalmound, a small city of Tanaw.
Okay, the bolded "when" could be "as" or "while", I think those words would work better there. Second, the underlined "small" can be removed, a stack of 24 units isn't really small. The "as" in italics should be "like" or might be rendered "The dwagons were blazing as bright as lava..." Also, the words "Chief Warlord" in blue should have the word "the" in front, so that's "commanded by the Chief Warlord of Avewnus".
The Tanaw were their eternal enemies. They fought with them for as long as Xarph can remember. And he was around for a few thousand turns..."Such a long time", thought Xarph. Raid to Offalmound was just a diversion. Xarph's stack was really strong. High level dwagons and knights, led by a level 9 warlord. It seemed like the main force.
Okay, the bolded sentence should be something like "They had fought with them for as long as Xarph could remember," the "had fought" is pluperfect tense--past relative to the past--and the could is past tense. The sentence in italics has the same tense trouble, it should be "And he had been around for a few thousand turns..." The word "Raid" in blue should be "The raid" or "This raid," whichever works better. And the word "really" in red is unnecessary, it can be removed.
Xarph didn't know the reason behind this attack. The main column even had an Abyssal Igniter. An enormous cannon, bigger than largest of dwagons. It's escorts were a small stack of Cownugon's, led by a Pit Fiend, about 30 Bwone Devils, 30 dwakes, 50 bezekiwa and 50 ewinyes. Avewnus' foolamancer accompanied that column,too.
The column's main target was Twelfetres. A level 4 city of Tanaw. Twelfetres had a very strong garrison.
The bolded words don't need apostrophes, that's an English issue mostly. The underlined "Avewnus'" should be "Avewnus's" unless Avewnus is a plural word. And the word "very" has the same problem as "really," it can just disappear without affecting the sentence's meaning.
Mawilith was the Tanaw's best ground unit, almost as strong as their cownugon. How could they possibly hope to defeat 150 of them? He didn't even know the Tanaw had so much mawiliths. He didn't know, and he didn't care. It was his duty to take Offalmound. He couldn't doubt his leader, he could only do his duty, and nothing more.
Now, the first sentence might be better rendered "The mawliths were Tanaw's best ground units, almost as strong as Avewnus's cownugon." The bolded "much" should be "many", when talking about a plural form, "many" will almost always be used.
Void Spear was a pitch-black polearm. It increased attack of every spear-wielding troop in its wielder's stack equal to wielder's level, including the wielder himself.
The words "Void Spear" should be "The Void Spear." Maybe the second sentence could also be: "It increased the attack of every spear-wielding troop in its wielder's stack by
the wielder's level, including the wielder himself."
Pit Fiend laughed, when he saw a flight of winged humanoids flying to Twelftres.
"Pit Fiend"-->"The Pit Fiend" Also, the comma in red can go.
It has just begun...
Avewnus was a level 5 city. It was in a small island in middle of a huge lava lake. It had 4 natural stone bridges connecting Avewnus to the mainland. Those bridges were a narrow, making Avewnus very hard to attack with ground units. Also, Hellflame Dwagons and Dwakes possessed the ability to swim in lava-lakes. They could act as "water" units in a lava lake, or as fliers. A river of lava which flowed through the region ended in this lake. Dwakes and dwagons gained
extra move and bonus to their defence while swimming in lava.
"were a narrow"-->"were narrow"
Last siege at Avewnus was a few hundred turns ago, when the Tanaw gained the upper hand in war and took most of Avewnus' cities.
But that siege was a mistake, and it turned the tide of the battle. Invading Tanaw forces were slaughtered by Pit Commander and his Fiends with ease. And those Fiends...Scary foes. They were fliers, physically stronger than their dwagons. In addition, they all had leadership. At level 3, they gained Shockamancy special. There were 7 Pit Fiends in Avewnus, 3 of them at level 3, 2 of them at level 4, and 2 at level 5. They rarely left Avewnus. Most of time, they were stacked with Pit Commander himself.
The three words or phrases in blue should be: "The last siege," "The invading," and "a Shockamancy."
Agrach woke up, and saw his rations pop. It was beginning of a new day, a new turn. He didn't get much action these days. He was only managing Avewnus and not doing much fighting. Xarph and Vis were always out raiding. Agrach wished that he was a flier. Avewnus' favored tactic was lots of small raids to multiple targets with fast fliers. Ground forces were mostly used for defence, and for big sieges, which were quite rare.
"Avewnus's"--the rule about the "s" being left off only applies to plural words that end in "s".
He was curious...Why did he have to manage the city? Aesa could have done it too, she was useless in combat anyway. But even she was out raiding somewhere. Agrach really wanted to join the main column. But Xarph said it was unnecessary. If it was unnecessary, why did a Pit Fiend marched? Agrach guessed that they needed a strong ground column. But since the column had no air, a Pit Fiend had been selected because of its wings to lead. It still seemed pointless to him. But well, Agrach knew the reason. He just didn't want to admit it. He didn't want to admit his failure.
My usual pet peeve is in red. The italicized sentence should be "If it was unnecessary, why did a Pit Fiend march?," the "did... march" acts as a single verb.
When he was fighting within his thoughts, walking through the city, he saw Vis take off from the tower and head south-west, taking fifty dwakes along.
"When"-->either "While" or "As". "When" tends to imply a completed action (When I'm done eating, I'll go to school.), and "While" and "As" imply more ongoing things (While I'm eating, I read a book.)
Avewnus possessed a good number of casters, worst of them mid-level. They had a Changemancer, Thinkamancer, Turnamancer, Dirtamancer and a Foolamancer, who was busy veiling their ground column which was marching to Twelftres. "Why?" thought Agrach, once more. He was going to get crazy thinking about it. But then he gave up thinking. He could never understand Pit Commanders plans anyway.
"worst"-->"the worst", "Pit Commanders plans"-->"Pit Commander's plans".
A while later, he saw the reason their archers gathered. From south, a massive ground column was coming.
"gathered"-->"had gathered". It's discussing the past, and you're writing in past tense, so you need a past-to-the-past tense. In English, the word "had" is used for that.
Overall, this is a good story. The concept is nice, I can imagine each of the units described, and the grasp of English is pretty good, despite all of my nitpicks. Keep writing, I'm interested to see how this story turns out!