Tripod is a system of customizing your character playstyle to your liking. Since you can only have up to 8 abilities at any time on your character panel, you are given the ability to customize your skills by putting skill points into them. You are given limited amount of skill points that you can put into abilities to change the performance of an ability with special modifiers. There are totally three tiers of Tripods for each skill. A new tier of Tripod is unlocked when a specified number of skills points are put into the skill and it reaches certain level. As an example we will use the very first basic skill of Deathblade called Surprise Attack which is just a quick slash attack.
When you put 4 points into a skill to reach Level 4, they get access to their Tier 1 Tripod: these are usually minor stat increases such as Increased Damage, Increased Range or Mana Cost Reduction. In this case Open Weakness will put a debuff on the enemy that will increase damage from your party members by 3% and increase damage of your front and back attacks by 9%, Vital Point Hit will increase Crit Chance of the ability by 15%, and Wide-Angle Attack will increase it's AoE range by 20%.
At level 7 after you put 20 point you will unlock respective Tier 2 Tripod: these ones usually add up a special effect to your skill, increasing the amount of hits they can do or changing their damage type entirely. In case of Surprise attack, Excellent Mobility increases the distance the attack goes, Impaired Mobility will slow down enemies hit by 40% for 4 seconds, and Keen Strike will increase ability's critical damage by 50%.
And finally, at level 10 when you put 48 points you will unlock Tier 3 Tripods: these tripods usually how the skill playes entirely. They can turn a single target skill into an AoE one, add a new attack that you can activate manually or automatically, create a new effect powerful effect at the end of your skill, and so on. In this case, Trailing Sword Burst will create an AoE zone in place where you have used the skill that will knock enemies backwards and inflict 100% additional damage but increase it's cooldown by 6 seconds, while Merciless Attack will change the skill into combo move and add a finishing hit after initial one and increase skill's total damage by 60%.
You can also enchance your abilities by socketing them with special runes. Runes are pretty straightforward and will make your skills more efficient, for example they'll increase damage done by the skill, reduce casting time of the skill, make it cost less mana, give a chance to reduce the skill's cooldown and so on. You can socket each skill by a single rune. Here's an example of some runes that are used in the late game.
And finally, you can progress your skills even further by using Armor Tripods. Starting Tier 3, the armor that'll drop to you will have three different Tripods attached to it for certain skills. They will increase the effectiveness of specified Tripods that are etched onto them and have a certain level, starting from level 2 and going up to level 5. These armor Tripods will increase the level of an active Tripod for the respective ability and increase it's effectiveness even further. Let's say my skill has a Tripod that will increase base damage to normal enemies by 50.0%. Tripod gear perk with each new level will increase that amount by 7.5%, making it 57.5% damage increase on Level 2, 65.0% damage increase on Level 3 and so on. You can hover over a Tripod in your skill menu to see what number will get upgraded, but remember - not all Tripods can be improved by using Tripod gear. As you acquire more and more gear with Tripods for your class, you will find powerful ones that you'll want to transfer to your relevant eqipped gear. For that you can transfer Armor Tripods between different pieces of equipment. It has a certain chance of success, and if by chance you fail, you will destroy the piece of equipment from which you are trying to transfer the Tripod. So it is a matter of farming for specific Tripods as much as possible and then transfering them to your current gear.
This is how tripods work: they change the way your skill works. It is important to play around with different tripods and find what suits certain style of gameplay that you are looking for. Depending on the skill and the Tripods you choose for them you can completely change their playstyle or purpose. Tripods are unlocked immediately and skill points are gained by completing various activities throughout the world, including leveling, completing quests, finishing Adventurer Atlas, or earning collectibles and exchanging them for Skill Potion rewards.