My answer from an other topic about nightmare builds:
"Assuming that your inquistor is the main tank in the party, I'd suggest the following:
Keep your equipment updated - craft new weapons and armor whenever its necesarry, and choose the trinkets and upgrades that increase health, constitution, defense or armor.
In combat, the things you need to focus on are getting enemy attention and keeping yourself alive by continously building your guard meter. A good tank build is based on defensive skills and taunting, so the main ability trees you need to focus on are Weapon & Shield and Vanguard.
From Weapon & Shield, Shieldwall is great because it builds guard and reduces the damage whenever this ability is active and you are hit. Passives are always useful, go for them. From the offensive abilites, Payback Strike is great and Shield Bash can be useful, but they are somewhat optional in terms of use, but you'll need them to unlock some passives.
From Vanguard you'll need War Cry as it taunts enemies and builds guard in the process. The other useful ability is Unbowed, as it also builds guard and it's most effective when you're surrounded by enemies. Again, go for passives, there're some great abilites like It'll cost you, which return a portion of inflicted damage to your enemies.
For a specialization, I'd recommend Champion (if you are a hardcore tank) but Templar is also a viable alternative, if you'd like to take a different approach and deal some damage too.
Let's don't forget about focus abilites, buy the perks which allow you to use high-tier focus skills, as they can really help you out when things are not going your way in combat or if you are looking for easily defeating your opponents.
For a party, I go with the "holy trinity + 1" squad, as it contains a warrior tank, a support mage and a dps rogue and one other characther of my liking, usually a rogue with a different weapon or a two-handed warrior. My most used party consists of Blackwall as two-handed warrior, Solas as the support mage and Sera as the archer dps rogue.
A two-handed Blackwall with his Champion specialization is capable of dealing tremendous ammounts of damage while ensuring his own survival, and is also capable of some off-tanking when I need to take the heat off my main tank or the enemies outnumber us greatly. He's gonna need Unbowed from Vanguard (and ofc. the passives) and a lots of passives from Champion. From the Two-Handed tree, get Block and Slash, Mighty Blow, Pommel Strike and optionally whirldwind (passives and upgrades are again, self explanatory). The Battlemaster tree contains some useful abilites too, grab Grappling chain because its fun and either or both of the Horns, but they're somewhat optional. This build also can be used for Iron Bull if you prefer him over Blackwall, with a small tweaks here and there, mostly specialization-wise.
As for Solas, his main strenght is in the Spirit (for buffing the party) and Rift Mage (for debuffing the enemy) trees. From Spirit, Barrier is a must and it should be used frequently, the two passives that come from it are also great, and I'd also suggest grabbing Dispel too, as it can remove hostile magic from your party, beneficial megic from enemies, dispel the barrier on enemies and kill demons before they're fully materialized from rifts, making your life easier. Rift Mage is great in weakening enemies, take everything from this tree. I'd also suggest some elemental abilites for dealing additional damage, mainly from the Winter tree, Winter's Grasp, Winter Stillness and optionally, Fade Step are recommended.
Sera is the ranged dps characther on the team, give her the best bow you can find take every ability from the Archery tree. Then focus on her specialization, Tempest and take everyting from there too, except the upgrade from Flask of Frost, as it taunts enemies and we don't wan't any attention on our rogue, as she's quite squishy. Thousand Cuts, the focus ability from this tree is marvelous and works wonders. And keep using the flasks.
Lastly, set your party tactics, so teammates would use defensive abilites automatically and frequently and disable the auto-use of those which require more attention, like AOE spells and abilites, which are most effective when a certain condition is met on an enemy (for example, they are stunned). Look out and try to go for combos, they can really increase your damage output. And don't forget to postion your party and check on them frequently, as they tend to move away to locations which are less advantagous for them in terms of survival. Make use of choke points and elevations (archers get a bonus when firing from elevated postiions) and let your warriors do the frontline work, keep your supporting party members back.
That's all I can think of now, I hope it helped."