It is not uncommon for a wizard of one school to be an expert in spells from others. Not only is there some crossover potential, there are also a lot of people who play multiples. However, for all the Wizard101 aficionados out there, not many have seen these spell cards.
In no particular order...
This seven-pip Balance spell is a boss's attempt to steal from the Elemental schools. First Fire got Heckhound, then wizards, through pets or otherwise, were able to obtain a Storm Hound and Ice Hound spell, though not in unlimited quantities. There has never been another hound for Spiritual schools, but now there's one for balance. At an average 96 damage per pip, it's not stellar, but it is unique.
This spell will introduce us to the structure of many mob attacks in game. It is X pips and does a particular amount of damage per pip, but has a maximum number of pips it can use. In this case, it's 90 damage per pip and up to three pips. Dark Mojo cast is carried as a natural attack by Zafarian mobs. Unlike Big Bad Hound, it's not really a spell that's cast, and is more of a natural attack.
#3 Feverish Chant
Not every boss attack has an image to go along with it, but the ones that do are pretty cool. Feverish Chant is one of those cards, and pictures a Grizzleheim raven. Unlike the mob spell Dark Mojo Cast, this one is not X pips. It's only one, but 150 damage per pip is pretty great value.
This is one of those spells you've probably actually seen in action before and just never caught the card. Flamesnake is a variation of Thunder Snake and Frost Snake for the Fire school which does 105-145 damage, which is identical in both value and range to Thunder Snake. Despite the low pip cost and similarity, this spell has a pretty cool animation and art piece.
With an average 605 damage, Lord of Atonement wouldn't be a particularly great spell at 8 pips, except for its AoE effect. Howling Chaney, the first boss in Darkmoor, introduces wizards to this spell, a glorified Forest Lord who pulls pillars from the sand rather than drawing energy from the trees of the forest and proceeds to beat his opponent with them. It's not only a pretty epic animation, but it wouldn't be a bad spell in the pockets of Balance wizards, either.
#6 Moon Trap
Remember those weird tri-blades and traps all the Celestian enemies used? Bosses and mobs alike enjoy Moon Blade, Moon Shield, and Moon Trap. You'll see this one a lot in particular in the Trial of the Spheres. It encompasses the three schools of magic most closely associated with the Moon school - Myth, Ice, and Death. All traps are 25%.
#7 Savage Mojo
The Elephants of Zafaria have their Dark Mojo and the Gorillas have their own - Savage Mojo. Savage Mojo not only follows the same general pattern as its sister spell, but also the damage amount. In fact, the only differences between the two are the name, image, and school. This one does Myth damage rather than Death. The art is still pretty great, though.
Serpent Strike is another mob attack with some awesome art, but it breaks all the different rules set forth so far. Firstly, it has a huge damage range, doing anywhere from 170 to 510 Fire damage. Here's the other catch - it costs X pips. That means that early on, it's an awesome investment, because you might get as much as 510 damage per pip, which would be off the charts. However, if you had gained a lot of pips, it would be a pretty poor choice. In fact, you could do so badly as to get only 12 damage per pip. It would be interesting to see KingsIsle add some trainable spells with this type of mechanic.
Shadowspear is a spell you've definitely seen in action, but probably haven't caught the card for. The three level 98 shadow spells are all attacks. Pleasing the form you choose over the course of several rounds results in the shadow casting a spell on you. If you please these spells at least two of the three rounds they have before attacking, they cast a Shadowspear. Perhaps as we get more Shadow spells, this will become trainable.
#10 Spicy Lightning Roll
This spell might sound like something you'd order at Taco Bell, but Spicy Lightning Roll definitely isn't something you eat. For a Storm spell at 2 pips, 190 damage isn't particularly great when compared with the minimum 105 from Thunder Snake, but the animation and name really provide a spotlight for the spell.
#11 Street Sweeper
You might recognize this image from a Storm version of a treasure card drop, but the original Myth version is likely one you haven't seen. Even with a Street Sweeper pet (simply named Frog), this spell is unavailable as an item card. It's a little bit of a unique card in that it doesn't appear to function like normal natural attacks - it looks like any old enemy could cast it and summon one.
#12 Thunderous Wave Mace
Ninety damage per pip on a Balance spell isn't bad, but it's the art that really makes Thunderous Wave Mace worth looking at. There are quite a few different swipes and hits with swords if you go looking on Wizard101 Central, but few are quite as epic as this one. Unfortunately, the name doesn't quite fit on the card.
Take Lord of Atonement and dial it up nearly 100 damage and you've got Lord of Blazes. Ever since Lord of Atonement, different versions of Forest Lord have been popping up in other places. A Death versions is available on the Death yeti pet and an Ice version on the Ice yeti pet. That gives us Life, Balance, Ice, Death, and now Fire for sure.
#14 Wood Construct
Similar to Street Sweeper, Wood Construct seems like it could be a natural attack, but it's also more of a summon. At 465 damage for 3 pips, it's pretty powerful even for a Storm spell, doing a guaranteed 155 damage per pip as compared to Storm Shark's average 135. And somehow, a Wood Construct doesn't seem quite as powerful as a Storm Shark.
#15 Monster Mash
If you don't know about this spell, then you're one of a fortunate few. Those who have run Darkmoor know it all too well and dread it. Not only is it incredible value for 6 pips, but the Beguile is absolutely brutal in a set of battles where keeping things quick, clean, and concise is essential. Many of the bosses carry this, and even some of the mobs. Besides just having a cool, if annoying, effect, it also has a pretty great animation. It combines Dr. Von's Monster and Vampire, much the same way that Catch of the Day, Sacred Charge, and Burning Rampage do.
I hope you learned something new today! Thanks for reading and see you in the Spiral! Credits: Huge thanks to Wizard101 Central Wiki Master Pyro for allowing the use of these card images!