Cassius Dio is actually a decent source regarding some of the more sordid details. He is also well known for the flamboyant speeches of generals.
There are many samples, but the battle of Pharsalus provides some detail...
From Book 41.60 (The entire battle takes place between ch. 52 - 63)
"And all this took place, as I said, not in one spot, but in many places at once, scattered all about, so that with some contending at a distance and others fighting at close quarters, this body smiting its opponents and that group being struck, one detachment fleeing and another pursuing, many infantry battles and many cavalry battles as well were to be seen. 5 Meanwhile many incredible things were taking place. One man after routing another would himself be turned to flight, and another who had avoided an opponent would in turn attack him. One soldier who had struck another would be wounded himself, and a second, who had fallen, would kill the enemy who stood over him. Many died without being wounded, and many when half dead kept on slaying. Some were glad and sang paeans, while the others were distressed and uttered lamentations, so that all places were filled with shouts and groans. The majority were thrown into confusion by this fact, for what was said was unintelligible to them, because of the confusion of nations and languages, and alarmed them greatly, and those who could understand one panother suffered a calamity many times worse; for in addition to their own misfortunes they could hear and at the same time see those of their neighbours."
Another... The Battle of Philippi, from Book 47.44
"For a long time there was pushing of shield against shield and thrusting with the sword, as they were at first cautiously looking for a chance to wound others without being wounded themselves, since they were as eager to save themselves as to slay their antagonists; but later, when their ardour increased and their rage was inflamed, they rushed together recklessly and paid no more attention to their own safety, but in their eagerness to destroy their adversaries would even throw away their own lives. Some cast away their shields and seizing hold of the foes facing them choked them by means of their helmets while they struck them in the back, or else tore away their armour and smote them on the breast. Others seized hold of the swords of their opponents, who were thus as good as unarmed, and then ran their own into their bodies; and some exposed a part of their own bodies to be wounded and thus gained a freer use of the rest. Some clutched their opponents in an embrace that prevented either one from striking and perished through the commingling of their swords and bodies. Some died of a single blow, others of many, and they neither were conscious of their wounds, since death forestalled their suffering, nor lamented their end, since they never reached the point of grieving. One who killed another thought in the excessive joy of the moment that he could never die; and whoever fell lost consciousness and had no knowledge of his state."
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