King Arboretum, referred to also as The Osierman and the Overking, was the last in the line of the Salix Dynasty and only Lord of Entinium. His rule is estimated to have been between seven and eight thousand years ago, predating every known human empire. He was the son of King Jorgk, and has often been attributed as a demigod, with some dwarven cults revering Arboretum as holy. His mother was said to be the nature goddess Saliceta, who was perhaps the personified entity of Entinium, the patron goddess of the dwarves, the world nymph or a tree spirit.

Much of what is known of Arboretum comes from oral tale, as the end of his life marked also the end of his kingdom. Arboretum has great association and affinity with nature, hence the speculation about his motherhood. His kingdom, (if it were a kingdom and not simply a citadel or fortress,) resided in the far north-western reaches of Engelain. Atypical for dwarves, it seems that Entinium held few architectural triumphs, as there has been few archaeological finds in the rough area, though perhaps a combination of time and looting has contributed to this loss.

Arboretum is known to be a conqueror, though how much of one has most likely been exaggerated as history has gone on. Arboretum was allegedly the descendant of petty kings which resided in the fortress-city of Salix, from which the name of their dynasty comes. The main power in the region at that time were the Westerelfs, who one day took it upon themselves to lay claim to Salix. Not only did they fail in their endeavour; they also filled Arboretum with righteous anger, who in retaliation took the entire kingdom of the Westerelfs as his own, never losing a single battle, and declaring the Kingdom of Entinium. Those elves which surrendered lived besides the dwarves as equals, though many more fled by boat to become the blasphemous descendants they are known as today.

The scale and size of Entinium is often debated. It presumably did not stretch as far southwards as the modern-day Free Kingdoms are situated, though whether or not it ate into the land that the gnomes currently call home is up for debate. There is speculation that a small sea empire was declared as Arboretum chased the Westerelfs from island to island in vain search of their vain king, and there are dwarven ruins on the isle of Haydrea to back this up.

What happened next is a matter for scholarly debate. Arboretum, who would be derided by the elves as the Osierman and remembered by the dwarves as the Overking for his embracing of the natural world above, had reached the pinnacle of his power. However, something then occurred, something lost to history, something that may never be discovered. Folklore say that Arboretum himself is to blame, that he had grown too proud, or that it was the treachery of the elves that still lived in Entinium which resulted in the cataclysm. Either way, all that is known to this day is that Arboretum disappeared without heir, and his kingdom fell into ruin.

There are some cults of Arboretum that have cropped up since his passing. There are those that believe he ascended to godhood himself, whilst others believe he simply died one day and this fact was forgotten by history. Regardless, as one of the most successful dwarven rulers to ever grace Engelain, his lost kingdom is a common pilgrimage site for dwarves- some believe that Entinium was in fact the birthplace of the dwarves, and treat the area as holy ground. There is a community of dwarves that live in the ruins of Salix, and many visit the Weeping Grove, which some believe to be the birthplace of Arboretum. There is a select few that reckon Arboretum did indeed leave behind an heir, who will one day reveal himself and retake the throne of old.