Complex sentence: Most of his forces were infantry, which he had formed into new units called "Coronelías," that were the seed of the later Tercios, armed with a mix of pikes, arquebuses and swords.
Split into simpler sentences: Most of his forces were infantry, which he had formed into new units called "Coronelías," that were the seed of the later Tercios. They were armed with a mix of pikes, arquebuses and swords.