@Willtheraven01
Why would he do that Twilight’s Kingdom? (That would also make his re-releasing them in Beginning of the End even dumber given how it backfired the first time.)

Also it was clearly established in Twilight’s Kingdom that Tirek escaped back in “It’s About Time” when Discord was still petrified.

@Incursor
I meant to say Release Tirek in Twilight’s kindgom.

IF you don’t believe me then in Beginning of the end part 1. Discord brought Tirek and Cozy Glow out of Taurus. Giving me a theory that Discord might have bring Tirek out of Taurus and join him on purpose to fool Twilight and the others.

@Willtheraven01
But Discord still tracked Tirek down to his general location before that point.

I’d say a single pony being drained wasn’t enough for Discord to notice (Tirek had to drain several by that point) unless he was actively looking for it.

I think he realize Tirek in the first place in that episode and he thinks heard a bang.

@Willtheraven01
Then how did he track down Tirek and react to Twilight getting the Princess’s magic in “Twilight’s Kingdom”?

Maybe he doesn’t have a magic sense after all.

@Background Pony #11C3  
Because the needed to make an example of Sombra to convince the rest of them they have to work together, thus jumpstarting said development.
 
@Background Pony #BD44  
Discord did it to prove they didn’t need the Tree/Elements of Harmony to use their power. And he didn’t know the Tree was sapient enough to count as murder.
 
@Background Pony #C1CC  
He’s far from the only character to forget about teleporting. At this point I think it safe to assume it has some unstated limit that prevents it from being used more often (like spur of the moment panic).
 
And we don’t know Discord didn’t not notice. He have had no reason to assume whatever they were doing their would be a threat to his plans as he was unaware they had Grogar’s Bell (due to his well established character flaw of being too arrogant to consider anyone could get the leg up on him).

I’m pretty sure Discord knew he stole it and let it slide.
 
It was only after Discord lost his magic (again) that he realized he f’d up

@soundtea  
Sombra on his own was a legit threat, so it makes me wonder why they needed to bring him back to go through the motions kill him all over. They already had the other three and we could have had more time developing their alliance and betrayal of discord/grogar and their bts workings in equestria.

yeah! it is so simple. you know discord was not gonna attack his friends as Grogar so it was better for him to be betrayed

@Background Pony #2302  
That whole little subplot was kind of half baked in general.

You know something, if Grogar was always Discord, then he is responsible for destroying the tree of harmony (I have a theory that Discord only cloned the King Sombra) … that makes him always the villain of the series all that time? Or was it just a coincidence?

@Background Pony #6ABB  
Good point, maybe he just assumed it would all work out in the end?

How do we know that his ability to sense magical imbalances, no matter how small, is something he automatically has up and running at all times? Even aside from the possibility that he pretended not to notice?

Not really

Well perhaps he did sense it but didn’t said anything since he is the guy that brought them back?

@Darth Shy  
Even if that was the case, he didn’t think to himself “Wait a minute, what’s Tirek here in Canterlot for?”?

Since he was Grogar, he probably knew about them being there and figured it would help in testing Twilight

Add on to that Tirek insisting that Discord’s magic couldn’t be controlled when he stole and used it in Twilight’s Kingdom.
 
Then there’s the fact that Discord couldn’t teleport himself out of the way of having his magic drained, that’s less of a plot hole and more of a lazy writing convenience.
 
Finally there’s Discord not keeping an eye on the three of them so he wouldn’t know that they found Grogar’s bell. It’s not that wasn’t possible, but it was more intentionally making Discord incompetent so the plot would progress unimpeded.
 
And those are just the ones I remember.

@Background Pony #6ABB  

 
Or maybe discord expected that. And went with it.

Writers gotta make Tirek, Chrysalis and Cozy Glow’s betrayal a surprise to Discord somehow.

Also for some reason ponies’ cutie marks didn’t vanish when Tirek steal their magic. And for some reason I saw a scene when Flash Magnus and Somnambula flew despite the fact, that Tirek stole pillars’ magic.

I…you got a point