The importance of Otto I, aka known as Otto the Great in the History of Germany, is something beyond question.

Born in 912 in the former state of “East Francia”, that is considered nowadays the first German state and whose lifespan was about a century, Otto the Great was the second king of the lineage “Ottonian” lineage.

He inherited the Duchy of Saxony and was crowned King of the Germans upon his father diedeath in 936.

He did a lot more than just retain the inheriteding his reign, h. He pushed forward the integration of the German tribes into a unified kingdom, a process that his father had begun.

Furthermore he achieved to behe status of crowned Emperor of the Germans in 962, as Charlemagne had done in 800.

The importance of Otto I, aka known as Otto the Great, in the History of Germany is something beyond question.

aka = also known as
aka known as = also known as known as

Born in 912 in the former state of “East Francia”, that i (which is nowadays considered nowadays the first German state) and whose lifespan wasliving for about a century, Otto the Great was the second king of the lineage “Ottonian”"Ottonian" dynasty.

Using commas for the "which is ... state" part makes the lifespan part feel out of place and hard to piece together with the rest of the sentence. Also, in context, it should be "with a lifespan of" or "living for," not "whose lifespan was."

He inherited the Duchy of Saxony and was crowned King of the German upon his father died's death in 936.

When his father died or upon his father's death.

He did a lot more than just retain the inherited reign, h. He pushed forward the integration of the German tribes into a unified kingdom, a process that his father had begun.

Furthermore, he achieved to bewas crowned Emperor of the Germans in 962, as Charlemagne had done in 800.

You can't use achieve with a verb like that.