Its Polymorph Time!
When I first saw Jalira, Master Polymorphist, I was determined to make a polymorph like deck with her.
So to understand this deck, you have to look at the combo that wins you the game. By having the only non-legendary creature in the deck be Worldspine Wurm, you are guaranteed to get to play it every time you activate Jalira. And once Worldspine Wurm is in play, you can sac him with Jalira to grab your other Worldspine Wurm, which effectively leaves you with three 5/5's and another Worldspine Wurm which you can use to sac again (since the other Worldspine Wurm that you sacrificed shuffled back into your library.) This lets you just keep sacking your Worldspine Wurms to make an unstoppable board presence that wins you the game.
Now there are other other ways to play other non-legendary creatures in the deck because they are not technically creatures. Cards like Raise the Alarm, Call of the Conclave, and Selesnya Charm allow you to have a presence on the field without interfering with your winning combo. (They also give you something to sacrifice to Jalira for the combo.) The other creatures that make up the deck are legendary creatures, so they dont screw up your combo either. Cards like Brimaz, King of Oreskos and Polukranos, World Eater. Both of these cards create a board presence by them selves, and can win you the game if Jalira, Master Polymorphist is nowhere to be found.
There are also some copies of Springleaf Drum in the deck so that we can use our tokens from things like Raise the Alarm and Call of the Conclave to Play Jalira, Master Polymorphist as early as turn 3, allowing the combo to go off on turn 4. Its are best option for ramp in the deck, since Elvish Mystic and Sylvan Caryatid are off the table.
We also find Chord of Calling in the deck in case we need to go get Jalira from our deck
Than we have Kiora, the Crashing Wave, which allows us to draw cards and play extra lands to ensure our combo works smooth and effective
Finally we have Detention Sphere, which is simply to slow down the opponents progress so that we have time to put the combo together.
A typical game might go like this
Turn 1: Land than Springleaf Drum
Turn 2: Land # 2 than either Call of the Conclave, Raise the Alarm, or Selesnya Charm
Turn 3: Land #3 and Tap one of your to creatures with Springleaf Drum to have 4 mana and play Jalira, Master Polymorphist
Turn 4: Land # 4 and activate Jalira, Master Polymorphist which grabs you a big ole Worldspine Wurm
Turn 5: Sac Worldspine Wurm to Jalira, Master Polymorphistwhich gets another Wurm and leaves you with 3 5/5's. Just keep doing this and attacking until you win.