You need to use the crystal skull to summon / unsummon your creature; no summoning spells are required. You can use the skull from your inventory or assign it to a hotkey / button.
A skull was first available in Broken Valley, to the far west, in the mine a little south of the fortune teller. The necromancer in the cave north of the destroyed chapel could have helped you create a creature. If you didn't get that skull, your necromancer in the battle tower would have given you one.
Trying to level in the fjords I had trouble with the first group of level 21 imps when I was level 17, but only needed another 4k experience to reach level 18. I found the creature much more helpful after I switched from a level 6 warrior head to a level 2 ranger one (didn't have a mage head at the time, but eventually switched to that class). It was more aggressive as a ranger/mage, and also made a better shield, since it didn't move around as much. In Broken Valley there are not as many rangers/mages, so a warrior creature can do fine, even if it is slow to react sometimes, and doesn't use Rush Attack that often.
Any alchemy formula in your inventory (not sure about the chest) when you talk to the alchemist to make potions (either in the battle tower, or the tree in the miller's cellar in Broken Valley) will be automatically added to the list of available potions. Any formula remaining are duplicates, and can be sold.
The same is true of enchantment formula and body parts for your creature. Any formula or parts brought to the alchemist/enchanter or necromancer in Broken Valley get carried over to the battle tower. I kept a list of the best creature parts I had, and sold any lesser ones before talking to the necromancer, to keep the parts list from becoming cluttered with new versions I wouldn't use.
