There are many differences between DS2 and DS3, which i can't explain to myself.
Battle between Zaramoth and Azunai
DS2: Zaramoth was Emperor of Stars. Battle between Zaramoth and Azunai was in time, when Empire of stars had existed.
DS3: By the legend Azunai was a human. He was born before Empire of stars existed. So... Was battle between Zaramoth and Azunai before founding of Empire of stars?
War with seck
DS2: Zaramoth created seck. And they took part in war against Azunai.
DS3: Seck bred giant spiders. Silk and eggs of those spiders were expensive at empire. Later they came to Ehb, legion defeated them.
DS1: Main quest was bound with returning of seck.
As I understand: By the lore of DS2 this returning of seck was after they were defeated in war with Zaramoth. By the lore of DS3 this returning of seck was after they were defeated in war, that was much later then the war with Zaramoth. But there is no disproof, that seck took part in wars after defeating of Zaramoth. Correct me, if i was wrong.
War of legions
DS2: When Emperor Veransk III died, Zaramoth became an emperor of stars. Thirteen legions bowed before their new emperor. But 10-th legion came to west, and nobody knows what happened to them (foundation of Ehb? ). With support of 13 legions Zaramoth became very powerful and the war of legions started. Those, who wanted the union of all lands of Aranna under one tyrant, followed Zaramoth. Those, who wanted freedom, resisted him.
DS3: The commander of 3d legion usurped the throne. Emperor was killed. But even in DS3 there are differences in different sources:
from first source: The legion was created in time of war, to restore peace in Empire. The commander of 3d legion usurped the throne, but then 3d legion was defeated and empire once again strengthened.
from second sores: At the time of Old Empire 10th legion was loyal to monarch. It was called the legion of emperor. (As I understand "Old Empire" was called the Empire before the war of legions. So did 10th legion exist before the war of legions or not?)
from third source: At the end of war of legions the third legion was defeated. Commander of 10th legion showed mercy and gave refuge to survivors in new kingdom of Ehb. (So "New Empire", "Empire, that once again strengthened" is kingdom of Ehb? Is it correct to call it like that?)
Plain of Tears
DS2: Before first cataclysm plain of tears was green and beautiful land known as Venanda. After Cataclysm it became desert.
DS2:BW: Kalrathia is located in the heart of plain of tears. After second cataclysm Kalrathia began to submerge to the water. Probably, at least part of plain of tears submerge to water.
DS3: At the time of war of legions, plain of tears was barren.
Lore of DS3 and DS2 are too different to be parts of one universe. . As i understend, sequence of historical events is:
in DS2: Empire of stars -> Battle between Zaramoth and Azunai -> Kingdom of Ehb -> Second Cataclysm
in DS3: Battle between Zaramoth and Azunai -> Empire of stars -> War of legions -> Kingdom of Ehb.
Stature of agallans
DS2: In DS2 we can learn the story of halfgiants' curse. Also we met agallans and halfgiant of first generation.
DS3:TOS: We can see agallans' corpses. This corpses are significantly lower then agalans we met in DS2. This corpses look like corpses of halfgiants of first generation.
There is no connection between DS2 and DS3 actions
There is no reference about first cataclysm
There is no reference about second cataclysm
At the end of DS:BW some heroes of DS2, like Taar and Amren, came to Ehb. But there are no references about them. (Also morden riders from quest in DS2 was going to come to Ehb. It is probably, that they died like other mordens in DS2:BW but there is a hope.)