First you have to injure your tentacle monster by for example the ones who are bad at scouting send them until they get injured. After that you should receive a skill to nurse/heal your tentacle and the next step is to build your royal army which you use for invading other surrounding territories.
you can get some items when you have conquered a land and you choose a tentacle monster who will train there. For example you can get some meat that can be given to tentacles who are hungry through bonding.