King of Sand was a savage beast that have eaten the flesh of many humans, magical beast, and other races. Unlike Wolfie, Its nature was bloodthirsty. Forming a normal contract with it was dangerous as it could turn on him at any time. However, he just had to use the beast contract method created for savage beast by beast tamers and everything will be fine.
In any case, making the life binding contract with it shouldn't pose any problems. Besides, having a peak D-rank savage magical beast as a companion also seemed impressive and will boost his combat ability by several times.
Doing as the Life Binding Contract method states, he stretched his hand to move it right above the Eighteen-legged centipede head and dripped a drop of blood mixed with ten units of soul force upon its mouth.
This was called soul blood.