"Ask Santa for the best present there is: a hail of destruction raining down on your enemies!"
The Santa's Surprise Spell creates Santa dropping down explosive presents on a certain area. It essentially serves as a much more powerful Lightning Spell with a larger damage radius, but with much less precision.
This spell was introduced during the 2012 winter theme update. When you cast the 2012 version spell, Santa comes and drops bombs in the form of presents on the defending village for a total of 1,500 damage.
It returned for the time period around Christmas 2016, albeit with heavily reduced damage, and was the first of three "Clashmas" gifts, the second was the Freeze Trap and the third was the Ice Wizard. It was available from UTC 10:00 23/12/16 through UTC 10:00 6/1/17. It has since made an annual appearance every Christmas period.
If the Santa's Surprise is used in multiplayer mode, it will generate up to five gifts on the opponent's village. These gifts spawn in a similar way as the tombstones, but each one is worth 5000 Elixir instead of 20.
Santa's Surprise functions in a nearly identical fashion to a weaker Lightning Spell.
Like the Lightning Spell, Santa's Surprise is most commonly used to eliminate problematic defenses. Two Santa's Surprise spells will take out a Mortar or a Wizard Tower, and three will take out an Air Defense.
The Lightning Spell is generally preferable over the Santa's Surprise spell, which deals less damage than the Lightning Spell and damages structures much slower.
The spell was available around Christmas 2012, but not on Christmas Day itself.
The 2012 version of Santa's Surprise took 1 day and 1 hour to make, which is 25 hours and Christmas is on December 25th. It used to cost 25,000 Gold, like all spells used to.
There was a potential cheat to win with Santa's Surprise. You could use 3 (1,500 x 3 = 4,500) Santa's Surprise spells to K.O. the opponent's Town Hall (at the time, a maxed Town Hall level 9 had 4,200 HP). If Gems were no object, it would be possible to gain significant amounts of Trophies quickly with this strategy (though it would take approximately 522 Gems to immediately brew all three spells each time, or US$4.10 per attack if bought in bulk, as a chest of gems used to cost $109.99).
Many players had speculated that there would be a chance that Santa's Surprise will come again in December 2013. However, it was not made available in the December 2013 update, which instead implemented the Santa Striketrap.
After the 2012 version became unavailable to create, anyone who had created the spell but had not used it would still have it in their Spell Factory. There were continuing reports on the forums (as of May 2013) of people being attacked by Santa's Surprise.
The same holds for the 2016 version.
If a player has the 2012 version and did not use it at all, it was automatically nerfed to the 2016 version.
The spell was added back in 2017 to be available until January 3rd, 2018. It retained its statistics from the 2016 version.
The Santa's Surprise made another appearance starting 24/12/18, and will be available up to 31/12/18. Due to the introduction of Town Hall 12 in June earlier in the year, and with the damage at maximum Town Hall level unchanged, the statistics for lower Town Hall levels were slightly nerfed as compared to 2017.
Santa's Surprise can't be donated to Clan Castles, due to being temporary.
Since the 19/12/16 update, the Santa's Surprise's stats scale in a unique way. While the spell is unable to be upgraded individually, its stats will automatically increase if you upgrade the Town Hall. Currently, for the 2018 version of the scaling, at Town Hall level n, Santa's Surprise does $ (\lfloor 30 + (n-1)*70/11 \rfloor)*9 $ total damage, with each strike doing $ (\lfloor 30 + (n-1)*70/11 \rfloor)*1.8 $ damage.
The Santa that appears upon using the 2016 version of the spell appears to resemble a Hog Rider. His "reindeer" resemble the Hogs that his brethren ride.
Clash of Clans official YouTube channel released a quick video in 2017 announcing the third release of the Santa's Surprise.