The talk of the Internet this week has been the unbanning of Bloodbraid Elf and Jace, the Mind Sculptor in Modern. There are lots of potential homes for each of them and many cards get better or worse based on what happens to the metagame as a result of the introduction of these two powerful cards into it. Today I'm piloting Reid Duke's Jund list with four copies of Bloodbraid Elf to see how it looks and feels in the most straightforward home. It's too early to tell how good Jund is, but Bloodbraid Elf certainly felt good in the archetype and will likely bring it back to at least on par with Abzan in terms of its power level and positioning in the metagame.
This is what I expect Jund to look like from now on in Modern. You don't have Deathrite Shaman like the old version did that dominated Modern in years past, but you do get some sweet additions in the form of Kolaghan's Command and Liliana, the Last Hope, each of which are great cards to cascade into with Bloodbraid Elf and each of which can return a fallen Bloodbraid Elf to hand.
In general, you want to cast Bloodbraid Elf when your opponent has a creature on board. This maximizes the utility of cards you can cascade into since hitting a removal spell when the opponent doesn't have a target for it makes Bloodbraid Elf only half as good as it should be. It's usually better to attack with a creature land instead of casting Bloodbraid Elf if a removal spell would have no targets.
Side out as many poor cascade hits as possible to minimize weak cascades. This means you may be better served by a sideboard that has fewer narrow cards and more generically useful cards across more matchups. This way you can board out more removal spells against decks with fewer targets. The cost is that your sideboard won't be as high-impact, but Bloodbraid can help make up that ground by finding your sideboard cards more consistently.
If your opponent has one card left in hand and you suspect it might be Cryptic Command, don't play out your last land in hand before using the +1 ability on Liliana of the Veil unless that land is super relevant. Otherwise they can respond to Liliana's ability by bouncing Liliana with Cryptic Command. Then, when her ability resolves you will have to discard Liliana since she is your only card in hand. Holding the land will play around that, even though it will cost you a land to do so.
Sometimes you want to kill your own Bloodbraid Elf in response to a Path to Exile, Detention Sphere or another exile effect. This will put your Bloodbraid Elf in the graveyard where you can return it to hand with Kolaghan's Command or Liliana, the Last Hope.
Ancient Grudge mostly only come in against Affinity, Lantern and other artifact-centric strategies. Aether Vial decks such as Humans or Merfolk generally aren't matchups where you want this. You already have Abrupt Decay, Maelstrom Pulse and Kolaghan's Command to kill their Aether Vials and this card risks making your Bloodbraid Elf cascades bad.
Fulminator Mage is a pretty flexible sideboard card that can come in against nearly anything, but it is best against Tron and Primeval Titan decks. It's also quite serviceable against decks with creature lands. Consider bringing in the extra Liliana, the Last Hope whenever these come in since that's another way to recur them.
Collective Brutality is mostly in the sideboard to combat Burn, but it also does a reasonable Duress impression against decks that are packed with instants and sorceries, especially if they have a card like Young Pyromancer that can die to the -2/-2 ability.
Grafdigger's Cage comes in against Collected Company decks and any deck trying to flashback cards from the graveyard or bring creatures back from the graveyard. I didn't board it in against Snapcaster Mage, but that was probably wrong. Nihil Spellbomb comes in against pretty much any deck trying to make use of the graveyard.
Grim Lavamancer is another that can come in whenever, but it's especially good against decks with a lot of creatures that die to a Shock. This is where you want to be against Aether Vial decks rather than Ancient Grudge. Thoughtseize comes in against the combo decks and against the slower control decks, including most Jace, the Mind Sculptor decks. You'll tend to want to board out creature removal whenever these come in, so it's an easy swap and you'll know when to make it.
Kitchen Finks is another card that can come in anywhere, but it's especially good against Burn. As a general rule, if you're not sure whether to bring it in against a midrange matchup, think about whether you'd rather cascade into Kitchen Finks or whatever other card you're considering leaving in over it. The answer will often be that you want to cascade into the Finks.
Lastly, if you're ever unsure of what play to make with Jund, imagine Willy Edel and Reid Duke standing over your shoulder telling you the right play to make.