The Sword of Omens is what its all about in the ThunderCat series (at least in
the first episodes). Conceiled in the hilt of the sword is the Eye of Thundera.
It's a magical / mystical juwel which holds many and varying powers.
In the first episodes we learn much of the sword and its powers when Lion-O is
taught the way how the sword works. Whenever he is in trouble for example he
uses the sword to call upon the other ThunderCats to help him out (the nowadays
classic ThunderCat yell). Lion-O discovers that he can use the sword for many
other things as well. It also gives him Sight beyond Sight. And there is of
course the nice thing that the sword will come to Lion-O's hand whenever he
calls it.
In the later series the sword gets more attention and they are also giving more
information on its past. Besides the Eye of Thundera there are more Thunderian
artifacts introduced; the Book of Omens being one of them.