Please explain this interaction

[Hermes_]

so, I saw the following three cards listed together on twitter and didn't get the interaction:

Sisay, Weatherlight Captain + Jegantha, the Wellspring and Intruder Alarm

[Mookie]

Sisay is the commander of a commander deck, with Jegantha as the companion.

If you have Sisay out, you can tap Jegantha to pay for Sisay's activation and tutor out a legendary permanent.
If you have Intruder Alarm out and that permanent is a creature, Intruder Alarm will untap Jegantha (and all other creatures), which means you can activate Sisay again.
Repeat until you have all the creatures in your deck on the battlefield (or at least all the ones you can tutor out with Sisay).

It's not quite infinite by itself (since you'll probably run out of creatures to fetch eventually), but it's still a strong enough interaction to win most games. And since two of the three cards effectively start in the opening hand, it's pretty easy to assemble the combo.

[Outcryqq]

With Intruder Alarm out: Tap Jegantha, the Wellspring, activate Sisay, Weatherlight Captain to search a legendary creature to put on the battlefield. This triggers intruder alarm, untapping Jegantha, and you can do this over and over again.

[Hawk]

For more specifics on what to search for:

- Kinnan, Bonder Prodigy lets you start getting infinite mana out of the loop, although do note that you can't use it for generic costs and likely need/want an extra mana dork.
- Purphoros, God of the Forge should allow you to kill without needing combat.
- Samut, Voice of Dissent gives all the legends you seek Haste to let you swing out for the kill (or generate infinite mana alongside a freshly tutored Rishkar, Peema Renegade, Selvala, Explorer Returned, or Radha, Heir to Keld)
- You can funnel that infinite mana into Kenrith, the Returned King, Golos, Tireless Pilgrim, and/or Shalai, Voice of Plenty for the win as well.
- Or, once you have infinite mana, you can start Sisaying out planeswalkers and win in any number of ways thanks to The Chain Veil and Oath of Teferi.
- I'm sure there's also some sort of infinite around some combination of Gerrard, Weatherlight Hero, Saffi Eriksdotter, Kethis, the Hidden Hand, and a sacrifice outlet like Grimgrin, Corpse-Born but looking at it makes my head hurt.

All that redundancy, alongside the aforementioned fact that you start with 2/3 combo pieces "in hand", makes the deck really resilient and powerful.

[Sinis]

All that redundancy, alongside the aforementioned fact that you start with 2/3 combo pieces "in hand", makes the deck really resilient and powerful.

On the bright side, people can see how close you are at the beginning of the game, and do their best to stop you.

[Feyd_Ruin]

What's the lowest card-count / density you can combo-win right away with?

Starting with only Sisay, Weatherlight Captain + Jegantha, the Wellspring + Intruder Alarm

The best I can come up with is 4 fetches:
(Keeping in mind Sisay's power for what she can actually fetch)

Fetch Rishkar, Peema Renegade, putting a counter on Rishkar and Sisay (5/5).
Fetch Aryel, Knight of Windgrace (Sissay @7/7)
Fetch Samut, Voice of Dissent
... Tap Rishkar and Sisay in response to untap
Fetch Purphoros, God of the Forge

Purphoros doesn't trigger Alarm (since it's not a creature), but with one of the floating you can start the final combo (and tap Jegantha in subsequent iterations, just for the )
Tap critters for mana, activate Aryel for token, shocking everyone with Purphoros.
Token triggers alarm to untap. Rinse & repeat for direct damage kill on everyone.

I feel there should be a denser method, with only 3 fetches.
Any ideas?

[pokken]

Sisay + jegantha wins with just
samut → derevi → najeela.

iirc.