I am in a 3.5e campaign and I am working on putting together a dragonborn knight that has really high defense, health, and can also buff adjacent allies. What should I look at for feats? All books in 3.5 are available for us to use.
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It is often said that the knight has five good levels: 1st, 2nd, 3rd, 4th, and 20th. Basically nothing in the class from 5th to 19th is very good. Loyal beyond death is amazing, but you are going to spend 16 class levels getting it, so keep that in mind.
Ultimately, because fighter and knight are so similar, this question about a fighter tank is very good reading. Since you are not a fighter, and do not get nearly so many (or such high-quality) bonus feats, you cannot do everything mentioned in the answers therein, but since the suggestions are mostly just feats and items, you have those options and just have to choose fewer of them. Dungeoncrashing is out, but that also lets you ignore bull rush related options; attacks of opportunity, tripping, and Intimidate all remain strong suggestions for you.
In particular, since the best thing you get out of increased knight levels is an increased saving throw DC for test of mettle, you should probably lean in to that and pump Charisma, and that also improves Intimidate. You don’t get the swift-action Intimidate that a Zhentarim soldier does, but the fearsome armor property from Drow of the Underdark can let you do it as a move action. Combine that with the Never Outnumbered skill trick from Complete Scoundrel and the Imperious Command feat, also from Drow of the Underdark, and you can reduce everyone around you to cowering at the start of every combat—while simultaneously challenging them to a test of mettle so even if they get over it, they still have to fight you. And you would still have your standard action to attack someone if you wanted.
Another potential use for Charisma is Use Magic Device—which you can make a class skill by taking the Apprentice feat from Player’s Handbook II. If your opening gambit in combat is to Intimidate everyone around you, test of mettle them, and also activate some useful wand, you’re in serious business. Most of the time you’ll probably have to move into position instead, but it’s well worth having the option.
Unfortunately, both Intimidate and test of mettle are mind-affecting effects (well, probably), which means if you are fighting constructs, undead, and similar mindless creatures, you could be effectively nullified. That’s where tripping and attacks of opportunity should come in. Unfortunately, as a pure knight, you have to take Combat Expertise to get Improved Trip, and that means Int 13, which hurts a lot when you want to focus on Charisma and Constitution, and not-bad Dexterity. You probably will have to dump Strength, which means you probably want Weapon Finesse (or a feycraft weapon—ask your DM if non-light weapons that nonetheless work with Weapon Finesse can benefit from feycraft the way light weapons do!). You could try to be a dragonborn killoren from Races of the Wild (dragonborn is a template, which means you also get a race to go with it) to take Charming the Arrow—you can actually use that in melee with one of the hybrid bows from Races of the Wild, but it’s probably not worth giving up the reach.
The suggestions of Mage Slayer, Occult Opportunist, and Supernatural Instincts recommended for the fighter tank are also good options for a knight. The Close-Quarters feats are probably too situational to bother with when you don’t have so many bonus feats. Martial Study and Martial Stance from Tome of Battle are also good ideas—you probably want defensive rebuke, and you definitely want thicket of blades. Island of blades and iron guard’s glare would be nice too, but you only get one stance at a time so be aware of that.
Finally, at very-high levels, you might want to buy a starmantle cloak from Book of Exalted Deeds. Nonmagical weapons that hit you while you wear that disintegrate. If you can dispel creatures’ equipment—there are weapon properties in Magic Item Compendium to do just that, or spellcasting allies can assist—you can destroy it all. On the other hand, though, it is 132,000 gp, potentially destroys your loot, and may ruin the fun of encounters, so think about it and maybe run it by your DM and group before doing it.