actually in south east asia, they called haki = Tenaga dalaman / ilmu batin.
People in south east asia like thailand, indonesia, malaysia and etc practice this power. I dont know if they really can do that power or not but just in traditional martial arts in South East Asia, they practice these powers.
Kenbunshoku ( Observation ) = Some of the shamans here can sense the dangers within the villages. The true events is when Japan Army tried to invade the village in Malaysia (1940+), shamans will tell the villagers to escape and run away or hide in their house because they can sense the presence or the army. This is just the rumors.
Busoshoku ( Armor ) = there are certain people in SEA who lived around 1930+ obtained this power. In Indonesia and Malaysia, they called it as Ilmu Kebal. These people will train and meditate in the jungle. Most of them were practising Seni Silat. martial art that used in movie Raid Redemption. The notable user = Taming Sari (1400+), Mat Komando (1900+). Also, this is just the rumors.
Then, there is another power that being practice by people in SEA. spiritual power where the spirit will doing job for the user. These spirit also will give or lend its power to the user. And, sometime, the ancestors will kept that spirit in certain things like Keris (weapon used by SEA people), Sword, Kerambit and Stick.
In 1969, there was a war between the races in Malaysia, and one of the group called Selendang Merah using flying sword to kill the enemy. rumors say, this thing happened because of the spiritual practice.
All of the above is just the rumors. but seems like similar to Haki.
So, my opinion is, maybe Asura is the spirit that being kept in sword by the previous user of Zoro's Sword (Wado Ichimonji maybe)