I believe in this answer that many of us are curious because it is one of the great mysteries of the game. All you can find are videos of people speculating on the subject, but even today there is no evidence on how to do this.
I will leave a few things on the subject if you want to go deeper or even be one more to try your luck.
"The first Warlord Sword entered the game early on. One of the Gods decided to sell it to the highest bidder at an auction. One player made the incredible bid of 300 diamonds (as these precious stones could be bought and sold for the same value it was a very common form of money since at the time there were no platinums or crystals coins). The player was able to use the sword for a short time until it and the player's entire depot was deleted by Durin after he discovered that all diamonds had entered the game illegally (possibly the player had found a way to duplicate some items).
Some time ago another Warlord Sword was found on Danubia. This sword was mysteriously on the floor of Mino Hell, in Rookgaard, and was picked up by a novice player who sold it at a very low price for not knowing its real value. The new owner used it for a while and then reported it to CipSoft GmbH, for having entered the game due to a bug. The sword was removed and the former owner was not rewarded."
Warlord Sword - Tibia Wiki