"I think the binding song (the slowing one) also closes/locks doors, but I might be misremembering."

It does. (I didn't kill Morgoth on my 1.3.0 win.)

I'm a little confused about how Poison works in Sil, or rather, how it differs between players and monsters.

As far as I can tell from the manual, if a werewolf bites you for 2d9 and you have no rPois, you take 3d9 to your poison counter, which decreases by 20% each turn, and you take that amount of damage (that the counter was reduced by). But I don't take any actual damage from the attack itself. On the other hand, if I have rPois I will only take 2d9 to my counter.

If I'm using a poison attack though, as far as I can tell, if I shoot a Longbow for 2d8 with poison arrows, I just hit the enemy for 3d8. My last character was hitting enemies for 60s with a big Slaying bonus and poison arrows.

I don't really see enemies taking poison damage over time, or succumbing to poison while running away from my poison arrows, which I would expect if it worked in exactly the same way for players and monsters. So what gives? Is player poison just an extra damage dice applied when you hit the enemy? Basically, do enemies have poison counters?

Poison brand vs. enemies works just like fire, cold, etc. It is the lowest-utility brand because resist poison is supposedly the most common "elemental" resist on monsters (I've never actually counted myself.)

Poison brand vs. player works exactly as you describe. A lemma to that is that poison resistance really doesn't do much against poison melee attacks -- it just shaves a die off the damage roll. It works much better vs "pure" poison damage like breaths or traps.

Arguably poison is better than fire in the late game. Morgoth is vulnerable to both, Balrogs resist both, fire drakes aren't poison resistant. "Cold" drake seems to be a name & little else.

Tolkien's cold drakes were just dragons that didn't breathe fire.

Only ancient spiders, spiders of gorgoroth, hithrauko, green S, and Hithrauko resist poison. Balrogs actually do not resist poison, I was surprised to see this. Only ancient emerald serpents among these are semi-dangerous. Compare that to "all balrogs" and red D's for fire...

Well, now I need to do a character with a poison brand weapon and see how it goes.

Huh, you're right about Balrogs, not sure why I thought they did. You missed adamant serpents but they resist all. I assume one of those hithrauko was meant to be kemen. Also wraiths of course but that only really matters for early game on an archer. So basically poison is better then fire past the depth of snow trolls & Gilim.

Yep. a (+1, 2d5)[+1] poison + sharp mithril longsword is pretty close to being Celeg Aithorn and with Danger is only 33 difficulty...

A venom greatspear is also really fun and relatively cheap, and lets you burn more points on making it lighter than Melkor. Crits + brand die on a greatspear are very exciting.

Yes, I've made a few of them before - but without the danger. Think I had a character dual-wielding sharp venomous mithril longswords.