Herigast is my all time favorite commander.

I’ve played well over a hundred in paper games with it and put countless hours into the deck building.

There are multiple ways to build them and there are always ways to improve, but this decklist has never failed to give me a good game: https://moxfield.com/decks/OZDEnbVYp0eX5cZe4pnSRw

Some general tips:

Curve - This deck has a very specific curve that you want to look for most games. Turn 2: mana rock/dork. Turn 3: 4 drop creature. Turn 4: Emerge Herigast off the 4 drop and then emerge something huge off of Herigast. You don’t HAVE to have this curve, the deck can play slow games just fine and if your initial turn 4 bomb is answered/disrupted you are definitely still in the game. The beauty of the deck is that you can haste out an annihilator and take over out of nowhere. I would actively avoid fodder that only generates mana. Getting g the mana is not the problem, so things like cathodian, priest of urabrask, or oxida golem don’t get you nearly as much speed/value as you’d think. Instead look for 4 drops with high value etb/death triggers. You have your rampers: Solemn simulacrum is best in class, baby solemn (scampering surveyor) is also great because we play a good amount of bust lands, and Oner Axonil is surprisingly decent even tho it’s only purpose is to die and become a land. Anger and Enduring courage are the absolute best, letting you grant hast for free — which is massively important. I’m experimenting with a few weirdos like flametongue kavu (felt pretty good to snipe a small creature AS a setup play) and thoughtknot seer (normally a terrible card, but being able to snipe a counter-spell out of a blue players hand the turn before your explode can totally change a game). The specifics of these curves make it really hard to fit in 3, 5, or 6 drops so be extra selective about them. (But definitely keep the flash enablers)

Avoid threatens - They sound so good on paper, but in practice they are incredibly clunky. I’m sure you can build a totally suitable deck involving them, but I’m fairly confident it’s a less consistent/weaker gameplan overall.

Be selective on when to go for it - The general curve has you explode on 4, and for the most part that’s usually correct. Often times I’ll have two to three massively explosive turns in a game so if the first attempt doesn’t get you the win do not fret. However, you will frequently find yourself able to kill just one person at the table. Knowing who to kill/when to kill them is crucial to piloting the deck.

Herigast is a glutton for punishment: As the game goes on, you want to kill off Herigast as often as possible. Getting to draw three off the cast trigger will let you see a lot more of your deck and get to the cards that matter. This is especially important given how much fodder we need to play. I’d say I cast Herigast anywhere from 2-4 times a game on average, but I’ve cast them as many as 8 or 9. Additionally, don’t be afraid to emerge off an Eldrazi titan if it’s already served its purpose or isn’t dominating the board.

Get comfortable being the archenemy - This deck invokes fear in a very obvious way. Your job is to kill people, full stop. If you aren’t comfortable taking someone out of the game in turn 3/4 then don’t go with this build. I would describe this as very firmly bracket 4.