Diablo 3 celebrated its second birthday last week. Here's why I'm still playing

The best thing about the black hole spell isn't the way it scrunches nearby enemies into an orb of writhing limbs, though that is nice, it's the way it positions an entire horde into the radius of a single meteor strike.

The best thing about the black hole spell isn't the way it scrunches nearby enemies into an orb of writhing limbs, though that is nice, it's the way it positions an entire horde into the radius of a single meteor strike. With two clicks I can drag a mob into a tiny space, and then scatter their broken ragdoll forms across the scenery with a massive explosion.

Yes, I'm playing Diablo 3 again. The lure of this week's 100% increased legendary drop rate—introduced to celebrate Diablo 3's anniversary—proved too great. I thought I'd just dip in and grab a couple of legendary items while the going was good, but idle curiosity quickly turned to fervour. Diablo 3's legendaries drop with a CLANG sound that elbows ambient battle noise aside. Legendaries fire a golden laser into the sky to announce their presence. The beam is to treasure hunters as the bat symbol is to Batman, a call of duty for obsessives seeking ever sweeter loot.

Diablo 3 is more moreish than ever, and I'm still trying to pin down its appeal. In part, I have to credit Blizzard's talent for creating innately satisfying audiovisual cues, but with the addition of adventure mode they've successfully broken the campaign down into a series of easily hurdled challenges. Complete five tasks in an act and you receive a chest, which contains keys that let you unlock Nephalem Rifts—the most entertaining and rewarding levels in the game.

Hearthstone has its own version of this absorbing structure. You complete daily challenges to earn currency, and then spend currency on keys to enter the arena. This is a survival mode in which you 'power up' your key with wins to unlock card packs and crafting materials. I wouldn't say that Diablo 3's progression structure makes it a better game—caving to compulsion isn't the same thing as being entertained—but those layered rewards generate a gentle sense of ongoing achievement that justifies repeat play, and the Nephalem Rifts are fantastic best-of mash-ups of Diablo 3's familiar zones. When I sit down for a half-hour of fun, a game that will leave me with a sense of having earned something will often win out, even if that something is a magic hammer that makes my virtual wizard's fireballs more explode harder. That's why Diablo 3 has dethroned Titanfall as my go-to action hit.

You can only successfully build that addictive crumb-trail of objectives around a satisfying core, and in Diablo that's theorycrafting and combat. To the outside observer who watches five minutes of a wizard blowing up goblins on the ramparts of act 3, the idea that a player can do that for 100 hours must seem like high madness. But imagine that the player revisiting that level isn't doing so to see a certain location again, or fight a certain combination of monsters, or even to earn another paragon level, imagine instead that they're testing a thesis.

Proposal: slotting a ruby into my weapon, which grants me a flat damage increase across all of my attacks, will give me a greater overall damage output per second than an emerald, which doubles the damage output of occasional critical hits. Testing methodology: raid the ramparts of Bastion's Keep, observe damage output and measure time taken to clear the area. Additional research: find out how the "damage" stat on my character sheet factors in sudden damage spikes caused by critical hits, and then note changes in the DPS measurement with both a ruby and an emerald equipped.

Welcome to the world of a Diablo nerd. Sometimes, there's huge satisfaction to be found in massaging a complicated mathematical system into a more streamlined form, especially when you're rewarded with the ability to melt goblins faster. Now consider the fact that there are five varieties of gem, each with 19 tiers. Then consider the five classes, with dozens of abilities, each of which can be modified with a series of runes, and accentuated with passive abilities. Diablo 3 took some stick for throwing out traditional skill trees in favour of a toybox approach that lets you reform your character at any point, but the ever-broadening array of options available to my characters makes them more fun to use as time passes. Since the Reaper of Souls update, it's even easier to gift a couple of amazing gems to a low-level alt and usher them up through the difficulties in no time at all. The game feels wide open in a way it previously wasn't.

Then there's the interaction between the character build and the tide of loot you collect. The conclusion to my gem problem, to illustrate the point, was that rubies gave me a greater damage increase. That's a false positive. In truth there's ultimately greater damage potential in a higher crit chance, boosted by a big expensive emerald. Now I prize items that improve my ability to crit, and use the enchanting system added by Reaper of Souls to try and turn almost-useful items into useful ones. The rabbit hole is deep and dark. Two years after launch, I'm still exploring.

Path of Exile is probably a better choice for theorycrafting purists, but I value the heavier feedback Diablo 3's fancier engine. I like to find an aesthetically satisfying combination of abilities, and then rework my build to support them. I recently replaced my Darth Sidious lighting spell with a move that hurls chunks of flaming rock through enemies. Each one leaves a smouldering trail in its wake. Between black holes and meteors, I bowl burning orbs across the battlefield to paint the floor with orange fire. I've found a combination of fire spells I enjoy, and favour item drops that improve their effectiveness, even if they mean a slight stat cut in other departments. I'm slowly building a fiery synergy across Diablo 3's interlinked skill and gear systems.

That's just with my wizard. Maybe I'll roll a crusader soon, opening another chapter in my Diablo 3 obsession. That won't be before the 100% legendary buff expires, mind. You have until tomorrow to jump in and reap the benefits.