This deck started out as a Reaper King deck with Scarecrows and Changelings (current version can be seen here:
Reaper King / Jegantha - Changeling Tribal. It was fun, and bad.
I gradually started adding Slivers, in order to power up the changelings, not planning on making an actual Sliver deck. But as Slivers began to comprise more and more of the deck, it became clear that it would just be much stronger as a legitimate Sliver deck. I had always wanted a Sliver deck anyways, and I was unexpectedly building one!
Thus, the bones of Reaper King were put back on the shelf, and I started experimenting with different Sliver commanders, eventually settling on Sliver Overlord. The deck was good enough, but I often felt like I was just top-decking Slivers and hoping for the best, as Sliver Overlord is slow and mana intensive, and it would frequently get removed before I could start using it.
Coincidentally, right around the time I was more or less finalizing that version of the deck, Modern Horizons was released, and with it came the The First Sliver, the best Sliver commander. It fit easily into the command zone, and the deck was immediately much more powerful. I also happened to have an unused Food Chain laying around, which fit right in as well, along with Eternal Scourge, Misthollow Griffin, and Squee, the Immortal.
Now, many The First Sliver decks use Food Chain and The First Sliver to cascade their deck out into some combo, but they are really just combo-control-goodstuff decks without any other Slivers at all! But all I wanted to do was just cascade-chain a bunch of Slivers out and swarm! There were plenty of Slivers in the deck, but a problem I kept encountering was stalling out by cascading into a non-Sliver card... I'd cast an expensive Sliver, anticipating an exciting sliver explosion, but after cascading once, I never thought I would be so disappointed to see a Sol Ring.
So I decided to start taking out non-Sliver cards to increase the chances of hitting a long Sliver cascade-chain, and it worked, so I ended up with just a few non-Sliver cards, including Enlightened Tutor (MIR), Worldly Tutor (MIR), and Vampiric Tutor (VIS) to search for Food Chain and the necessary creatures. And here's where it clicked that when cascading off a 2 mana-value sliver, I would always hit either one of the these three tutors, or one of the three 1 mana-value slivers - Striking Sliver, Galerider Sliver, or Virulent Sliver. While these three Slivers are very good, I found myself always hoping to hit a tutor instead, so I took the three Slivers out! This now meant that when cascading off a 2 mana-value sliver, Food Chain could always end up going on top of the library, because I would only ever hit a tutor! With this realization, I ended up taking out Eternal Scourge and Misthollow Griffin, as they were not slivers, and now I could much more consistently tutor for or cascade into Food Chain or Squee, the Immortal, Squee being the strongest of the three Food Chain creatures.
Following this line of thought, I also took out all of the non-sliver 2 mana-value cards except for Demonic Tutor (LEB). Now, cascading off a 2 or 3 mana-value sliver would ALWAYS tutor Food Chain or Squee to either the top of the library or hand! Furthermore, the only thing stopping a 4 mana-value sliver from ALWAYS cascading into a tutor would be 3 mana-value non-sliver cards, like Food Chain and Squee, the Immortal. From here, it was easy to take out all other non-sliver 3 and 4 mana-value cards so that almost ANY Sliver would ALWAYS end up cascading into Food Chain, Squee, or a tutor!
I've eventually replaced Enlightened and Worldly Tutor with Imperial Seal. Sometimes these other tutors wiff, in as much as they don't always get the card you need, and Imperial Seal does. With Modern Horizons 2, I added Profane Tutor.
So this is how the deck stayed for a while, and it was good... However, the two biggest problems it had seemed related, and they revolved around a lack of ramp, and the fact that after casting The First Sliver, if I didn't have enough mana to cast a second Sliver to start the cascade chain that turn, I would have to wait until my next turn to cascade, and if The First Sliver did not survive until my next turn, it set me back a lot. I found myself waiting until turns seven or eight or later to cast The First Sliver with enough mana to be able to cast a second sliver that same turn. But I usually still had to wait another round or two to attempt a combo, while holding up Slivers in my hand to trigger cascade, and still hoping that The First Sliver would survive.
The only real ramp in the deck was Manaweft Sliver, and Gemhide Sliver. I also had Cryptolith Rite in for a while but have taken it out).
Enter Jegantha, the Wellspring.
At this point, most of the cards in the deck already met Jegantha's companion requirement, with notable exceptions being Telekinetic Sliver, Kindred Discovery, Kindred Summons, and Squee, the Immortal. Telekinetic Sliver and the Kindred spells are great cards, and I am sad to see them go. Eternal Scourge, while far inferior to Squee, was an easy replacement, and was easier to cast.
With Jegantha as the companion, Jegantha can be cast before casting The First Sliver. This way, if Jegantha ended up dying before my next turn, that's too bad, but at least it wasn't The First Sliver. If Jegantha does survive, I can cast The First Sliver using Jegantha's mana, leaving all my lands untapped to cast a second Sliver to start the cascade-chain on the same turn as casting the First Sliver. With out enough mana to cast The First Sliver and play Sliver spells, I'd have to wait another turn anyways with The First Sliver vulnerable to removal, but now Jegantha can stand in for The First Sliver during that first turn, and provide a bunch of extra mana the next.
I'm not sure when I decided to put in Intruder Alarm, or really why I thought of it. I guess I was just brainstorming about other strategies I could include, alternate win conditions. Intruder Alarm takes 3 cards to combo, while Food Chain just takes 2, but I think Intruder Alarm can be a little easier to get online. This card is explained more below.
Because this deck has such little ramp in it, but is still mana hungry even with Jegantha companion, I've included 40 lands, and I still miss land drops!
And that's how this deck got to where it is now. Even though this deck did become a combo deck, using Food Chain and Intruder Alarm to cascade out the deck, it still just cascades out a bunch of Slivers and swarm!
