I'm about to hit level 3 in Descent into Avernus and am thinking Horizon Walker or Beast Slayer. How useful is it to be able to detect portals vs using hunter's sense to detect immunities/ resistance/ vulnerabilities? Is detecting portals basically useless, but a trade for the extra d8 damage (vs d6 for beast slayer) and eventual teleport abilities? Playing an Aarockokra archer.

Most of your fiends are going to have the same immunities and resistances, just by their type, but it's always good to know when there's a curveball. It's also useful if someone is disguised -- ping that suspicious dwarven salesman. He doesn't have resistance to poison? Something's up. My ranger's uncovered multiple disguised baddies this way. 3rd level Monster Slayer also gets you Slayer's Prey, which is a free d6 damage per round (not per attack, like Hunter's Mark) and your 7th level ability (and 15th, but I don't think Avernus goes that far) build on it.

Detecting planar portals is nice, but are you going to be able to use them? Also it's once per rest. It's going to be not useful most of the time, but incredibly clutch when it is. Planar Warrior, the bonus damage you do at 3rd level, takes your bonus action every round, it doesn't stick like Slayer's Prey and Hunter's Mark do. But the conversion to force damage is pretty useful, since unless you have a magical weapon, your weapon damage will be halved, but immunity or resistance to force damage is pretty rare. I'd vote for Monster Slayer if you have a magic weapon, and if you don't, it's more either-or.

Thanks for that- never occurred to me to use the hunter's sense that way. Plus highlights need for a good weapon or the Slayers extra damage might not help much agaibst some enemies. One thing I still wonder about is the utility of the spells offered- do you find yourself using the Slayer's Zone of Truth or Magic Circle at all? Vs I can see using the Horizon Walker's Misty Step and Haste pretty consistently.

As an aarakocra with a ranged weapon, you're never going to need movement buffs. Closest you ever need to be to a target is 60 feet for the setup rounds, then you can be 90 feet or a couple hundred feet, if you have sharpshooter. Your flight speed is 50, which is 20' further than most walking races -- if there are a lot of flyers in Avernus, that might change things a little, but just take zephyr strike and you're golden. Magic Circle has the problem of a 1-minute casting time, so it's really only for combat you can prep for. Zone of Truth I've used a couple times. My concentration is mostly used for Hunter's Mark (prior to the CFV UA, now I don't need concentration for that) or Protection from Evil and Good or Healing Spirit (as a last minute usage for someone rolling death saves, with my movement I can usually reach someone our bard can't reach with healing word).

Thanks! You're the best.

Ok. Think I'm convinced. Monster Slayer it is. I had taken Zephyr Strike at lvl 2 and yeah, for a flying character it has left me incredibly overpowered if I decide to swoop in for a quick melee attack then zoom out to use the bow.