Living Items are special items which automatically adjust to the level of the character using them. They can be purchased or acquired only through special event promotions. Stormblade is offered for you by thr offer Stormblade deal and the Stormgun is offered with the offer referred as Stormgun deal.
Currently the only obtainable items of this kind is the Stormblade and Stormgun (another living item is Skybreaker, which was used in 2015 as the special promotion and second is Ironbound Cape, offered in April 2017) is offered shortly after a new player begins playing (after completing level 3) and costs 99 cents. The offer is limited time, limit one, and is not ever offered again. Stormblade is focused to be given to Warrior and Mage, as on the other side, Stormgun is only usable and buyable from Bounty Hunter class. Whenever a player's first character eligible for one of these items reaches level 3, the offer becomes available for a limited time, after which there is no way to get it again, or get the offer back if you did not take it.
You don't have ensured socket when buying this living item. However, you can buy the additional one by limited cost of Gems (the cost is lower when at the lower levels of your hero class).
Also note, that you can only get epic, not the legendary or any another quality types. However, the special bonus is the same as it would be on legendary item.
If the level cap is increased, your named item will be again leveling up with your hero to new max level that can be obtained (to be able to level up the sword you have to use it in the current character you are fighting with, not another hero you have created).
All Living Items gives a certain bonus that is affected by their type and quality. Meaning that the bonus that is able to stack is increased by each level the item obtains.