Summon Bolt (招雷, Shōrai) is an effect introduced in BS25 that is currently exclusive to Blue Spirits.
The effect name is probably a play on the word, 招来 (the pronunciation is also Shōrai), which means "calling someone/something". In the lore, it is being referred to as a tactic called "Calling Thunder", a transliteration of the keyword's name, and is a key part in sending the investigators to the Reverse Continent, and later bringing them back from the past.
Spirits with this effect have the following text:
- Summon Bolt: Cost X (When Battles)
At the end of the battle, by destroying this Spirit, summon a Cost X Spirit card in the same family as this Spirit from your Hand, without paying it's cost.
In essence, the keyword allows the player to summon a different Spirit at the end of battle by paying the price of sacrificing the Spirit that has finished the battle. This means that the player can summon a new attacker/blocker after a battle is finished, and use the effect to summon big Spirits with high costs at ease. Limitation of the effect, however, includes how the Spirit must survive until the end of battle for the effect to proc, and it requires the correct summon target (of the designated cost) to be in the player's Hand. While it makes good use of Blue's hand-filter ability, destroying own Spirits is also a significant loss to Blue which doesn't usually have lots of Spirits on board due to a limitation of hand size. Hence, supporting cards of Summon Bolt revolves around helping the Spirits with such effects to survive until the end of battle, activating the effect without destroying the Spirit, and returning cards from Trash to Hand to compensate the loss.
While introduced in the Ultimate era, it received a powerful upgrade in Burning Soul era with The Peafowl WisdomKing which targets Cost 2~8 instead of the usual one or two specific costs. Additionally, being a Burst that comes with draws with net gain, it allows the player to construct their deck using the keyword more effectively. Since the keyword is said to be a technology owned by the Pirates in the Ultimate era, it has long been a keyword exclusive to the Fighting Spirit family, with additional families like Fencer or Astral Guardian. Since family is referenced for the summon target, the family owned by the Spirits with Summon Bolt determines the range of summon targets, and the combo range of cards. The addition of Vassal, a major family in Burning Soul era, with The Peafowl WisdomKing, adds a big card pool for combo to the deck, or deck that uses Summon Bolt as part of its engine. Since color is not restricted, players can summon Red Vassal family Spirits without paying the cost via The Peafowl WisdomKing's Summon Bolt. Later, in Ultra Advent Era, with the introduction of The GrandseaPirateCaptain Narwilliam, Worldswalker and Fusion Beast, one being the major era family of the time, the other being another big staple family in Blue besides Fighting Spirit, are added to the pool, and have expanded the possibility further. However, since Narvilliam himself is not Fighting Spirit, it can't be linked to the previous Summon Bolt Spirits.