This is an entry level Trials Build, so it doesn't use the Best in Slot sets. It's intended to Get you the Best in Slot sets, by dungeon running with a more experienced Squad, that doesn't do gear-checks. Then, maybe you can work your way up to competetive trials, for the Perfected sets, but you have to start somewhere:
FAQs: "Why a Templar? Wardens are the best in Slot now, Templars are passe." Okay, first of all, you have to be an ESO Plus member, or buy the Morrowind expansion to even play a Warden. This is a more affordable (Less Pay to Win) option to get your feet wet. Also, they offer rare-to-unique benefits you can't really get in any other class, which extend to your entire Squad, or Fire-team. (A smaller division of a Squad. In a 12 man Group, you have a main Battle tank, then sub-division when you have to split up, to open doors, or whatever)
This is part of the real-world Tank design rochambeaux. Take the T-34 line, they have a good speed, and give up a little Armor/Firepower to get it. M=FP/A, where Mobility equals Firepower over Armor. Only in this case, you're not giving up Armor, you're giving up Health (I guess analogous to Crew in the tank analogy) for Magicka, and Stamina to grant Mobility to your allies. Also Spell Slots you could be using for Firepower, or Damage Shields, instead.
Radiant Aura+Retreating Maneuver: Minor Expedition+Major Expedition, to everyone around you. Also, Minor Magickasteal for:
Elemental Drain: Your Frost Staff is also a Destruction Staff, so after you pick 1 target to Focus, anyone that hits them, including you, gets Magicka Back. Now, you can charge a Power Attack (Elemental Drain does not generate Aggro. Mobs don't even notice) you can taunt them, with Magicka back, and then charge into the fray with your melee DPS, and the Main Battle Tank. Radiant Aura also gives you, and them Minor Magickasteal to EVERYONE in the melee. As long as you hold it, to hit everyone in the melee. Then, when they're done, you can cast Rapids/Aura for a 45% Speed boost to the entire party, inside your AoE. For Time triels, this translates to a better Time on your speed run, and higher net DPS, because you spend more time fighting, and less time running to the next fight. (If you have a Nightblade, they can even cast Bolstering Darkness on top of that, which more than Doubles the whole team's movement speed, with a Synergy.)
Power of the Light: I put this on the Shield side, because it's Stamina, and it maps right over Elemental Drain on the Staff Side. Only this one stacks Minor Fracture, and Minor Breach on top of Major Fractur/Breach (As well as major/Minor Magickasteal.) This increases the Firepower for the DPS, on the same target. Mostly reserved for the Boss, mini-boss, and high priority adds, like the Healer. (A boss fight is that much harder, when the Boss has a Healer. Kill them first, so the Tank doesn't have to worry about them.)
Armor: Or Damage mitigation, but here you're going to be a little behind a Dragon Knight. They have Obsidian Shield, you don't. However, you have Luminus Shards, which grants them the Resources they need to Block (Stamina, magicka, and Health) So, drop a Spear on the MBT, he'll appreciate it when he picks it up. Remember, you're not The tank, you're there to Support The Tank.
Not Undaunted Synergies, but this is Internal Synergy, that makes the build work, regardless of the Gear Sets. (I'll get to those.)
1H%Shield works with the Templar Class, because it has effects that duplicates Class Spells, with Stamina. So, you can map the same basic effects to the same buttons, on both bars. The difference is, on the Destruction Staff side, you use Magicka, while your Stamina recovers. Then, you switch to the Shield, and use Stamina, when the enemies are already affected by Magickasteal. This means that you don't have to worry about Magicka Recovery. Your sets, glyphs, armor, and even Race choice (Redguard/Imperial) can all be focused on Stamina, and Stamina Recovery. You don't have to stay on the Magicka bar, which doesn't have to be very long, and spell damage/critical/penetration doesn't really affect you. You put Spell Penetration on the mobs, so the MDPS can kill them faster. That's 1/3 of your function in the group: Supporting the Artillery.
 Shielded Assault/Toppling Charge: This isn't Just a Gap Closer, it's also a Stun, and a Buff/Interrupt for you. Since the latest patch, there's no minimum distance, so even standing Shield-To-Toe with that miniboss, you can pop that little bubble on Shielded Assault. Yeah, it's not as big as Obsidian Shield, or even Radiant Ward. It doesn't have to be, because it's cheap, and it lands with a Stun. So, when you're swarmed by Anklebiters, you can pass out Taunts, and Stuns, every time your bubble bursts, you get it right back, passing out taunts, and Stuns.
 Inner Fire/Ransack: Again, Spamina/Magicka. I still use the Fully Charged Power Attack, because it basically refunds the cost of casting Inner Fire. Not to mention:
 Elemental Drain/Power of the Light: This is how I get away with using Ransack instead of Pierce Armor. Minor Breach/Fracture on top of Major Breach from Elemental Drain, and Major Fracture from Ransack. On 1 target, again pick 1 target, to die (And gain Magicka back from) First, that one. The Healer is always my priority target, but then Ranged, or Chargers who can run away to shoot from outside the death zone, or go chew on the healer instead of my Shield. Once they die, pick another one, and another one, until the pack is cleared.
Then, back to The Boss, check on his Status. (You know what Elemental Drain looks like, right?) He's almost guaranteed to have Major Fracture on, from the Tank keeping him Punctured. What about Minor Breach, and Fracture?
Backlash: Here's where we get nasty. When Power of the Light times out, it deals Physical damage. However, this damage isn't scaled off of Your stamina, agicka, weapon damage, spell damage (Though your Critical Damage does count, if it Crits, you're not a Khajiit with the Shadow) It scales off the DPS. All of the Damage Per Seccond, for every second that it's on him. 20% of that, 20% of the total output of all the DPS in the squad, on that target, all at once. It's essentially a 20% DPS boost, all at once.
So yeah. Firepower.