1. Destroying Tamriel
Kaalgrontiid and the aeonstones have enough power to destroy Tamriel, but Abnur Tharn and Nahfahlaar reduce it to just island level. The area of Tamriel is given in the Arena manual as 12,000,000km^2 (or 1.2e+13m^2).
http://tes.riotpixels.com/arena/files/arena-manual.pdf
Arena Manual said:
If you are near the edge of one region the automap may not show you the next region until you enter it. This is not a mistake in your automap, but a memory limitation of the computer. To hold 12 million square kilometers of maps would take more memory than most machines have!
Next, we need to find the average elevation of Tamriel. I would have just gone with a low end of the lowest elevation in the world, but it's blatantly clear Tamriel has a higher elevation than the Maldives, so we'll get an average of all the countries average elevations.
https://www.atlasbig.com/en-us/countries-average-elevation
H = 1.8 + 15 + 28 + 30 + 34 + 34 + 61 + 69 + 70 + 83 + 85 + 87 + 91 + 108 + 108 + 109 + 110 + 118 + 126 + 139 + 143 + 149 + 160 + 160 + 162 + 164 + 173 + 173 + 175 + 178 + 181 + 190 + 207 + 228 + 230 + 236 + 243 + 246 + 246 + 250 + 263 + 273 + 276 + 279 + 282 + 287 + 297 + 298 + 305 + 310 + 312 + 320 + 320 + 321 + 325 + 330 + 331 + 340 + 343 + 345 + 360 + 367 + 372 + 375 + 377 + 380 + 384 + 387 + 388 + 398 + 400 + 410 + 414 + 423 + 424 + 430 + 430 + 433 + 438 + 442 + 442 + 442 + 450 + 458 + 460 + 470 + 472 + 472 + 473 + 474 + 478 + 487 + 492 + 498 + 500 + 508 + 514 + 538 + 538 + 543 + 557 + 568 + 577 + 593 + 595 + 600 + 635 + 660 + 665 + 667 + 667 + 684 + 702 + 708 + 710 + 726 + 741 + 746 + 759 + 760 + 762 + 779 + 800 + 812 + 853 + 900 + 909 + 910 + 961 + 999 + 1,013 + 1,018 + 1,034 + 1,086 + 1,111 + 1,112 + 1,117 + 1,132 + 1,138 + 1,141 + 1,150 + 1,192 + 1,250 + 1,305 + 1,330 + 1,350 + 1,432 + 1,504 + 1,528 + 1,555 + 1,598 + 1,792 + 1,792 + 1,840 + 1,871 + 1,885 + 1,996 + 2,161 + 2,988 + 3,186 + 3,265 + 3,280
= 103455.8m/162
= 638.616049m
V = 1.2e+13 X 638.616049
= 7.66339259e15m^3
Now for our energy. A low end of violent fragmentation (69000000 joules per m^3) and a high end of pulverization (200000000 joules per m^3).
(Low end)
E = 7.66339259e15 X 69000000
= 5.28774089e23 joules
= 126.38004039196941619 teratons
(High end)
E = 7.66339259e15 X 200000000
= 1.53267852e24 joules
= 366.31895793499046476 teratons
Abnur Tharn and Nahfahlaar restrain this explosion, and make it so only Dragonhold is destroyed.
2. Levitating Dragonhold
A league is equal to 5.55600km, or 5556m. It's hard to say where the island was located, but going to the bottom of the screen would get us a minimum distance.
753 pixels = 5556m
1 pixel = 5556m/753 = 7.37848606m
7.37848606m X 290 = 2139.76096m
2139.76096m/2 = 1069.88048m
7.37848606m X 76 = 560.764941m
7.37848606m X 82 = 605.035857m
605.035857m/2 = 302.517928m
7.37848606m X 58 = 427.952191m
7.37848606m X 79 = 582.900399m
582.900399m/2 = 291.450199m
7.37848606m X 64 = 472.223108m
7.37848606m X 111 = 819.011953m
819.011953m/2 = 409.505976m
7.37848606m X 109 = 804.254981m
7.37848606m X 96 = 708.334662m
708.334662m/2 = 354.167331m
7.37848606m X 103 = 759.984064m
759.984064m/2 = 379.992032m
7.37848606m X 115 = 848.525897m
object degree size = 2*atan(Object_Size/(Panel_Height/tan(70/2)))
= 2*atan(290/(1045/tan(70/2)))
= 0.261478453 rad
= 14.9816117905470225 degrees
Put that into
the angscaler, and Dragonhold is a distance of 8136.6km. We'll pocket that for later...
Volume of everything as cones.
V = πr^2h/3
= π X 1069.88048^2 X 560.764941/3
= 672171353m^3
= 6.72171353e+14cm^3
V = 6.72171353e+14 X 2
= 1.34434271e15cm^3
V = πr^2h/3
= π X 302.517928^2 X 427.952191/3
= 41013431.4m^3
= 4.10134314e+13cm^3
V = πr^2h/3
= π X 291.450199^2 X 472.223108/3
= 42005345.9m^3
= 4.20053459e+13cm^3
V = πr^2h/3
= π X 409.505976^2 X 804.254981/3
= 141235173m^3
= 1.41235173e+14cm^3
V = πr^2h/3
= π X 354.167331^2 X 708.334662/3
= 93043086.7m^3
= 9.30430867e+13cm^3
V = πr^2h/3
= π X 379.992032^2 X 848.525897/3
= 128304740m^3
= 1.2830474e+14cm^3
Now we'll have to get the energy for this, using GPE. Assuming Nirn is Earth-sized, it would have a gravitational pull of 9.807m/s², which we'll times with the mass and the centre of gravity. The centre of gravity on a cone is a quarter of the way from the base.
The average density of continental crust is 2.83g per cm^3. However, given each island is different in size, we'll have to find it for each island individually...
H = 560.764941 X 0.75
= 420.573706m
M = 6.72171353e+14 X 2.83
= 1.90224493e15g
= 1902244930000kg
E = 420.573706 X 9.807 X 1902244930000
= 7.8459354e15 X 2
= 1.56918708e16 joules
H = 427.952191 X 0.75
= 320.964143m
M = 4.10134314e+13 X 2.83
= 1.16068011e14g
= 116068011000kg
E = 320.964143 X 9.807 X 116068011000
= 3.65346739e14 joules
H = 472.223108 X 0.75
= 354.167331m
M = 4.20053459e+13 X 2.83
= 1.18875129e14g
= 118875129000kg
E = 354.167331 X 9.807 X 118875129000
= 4.12891246e14 joules
H = 409.505976 X 0.75
= 307.129482m
M = 1.41235173e+14 X 2.83
= 3.9969554e14g
= 399695540000kg
E = 307.129482 X 9.807 X 399695540000
= 1.20389049e15 joules
H = 354.167331 X 0.75
= 265.625498m
M = 9.30430867e+13 X 2.83
= 2.63311935e14g
= 263311935000kg
E = 265.625498 X 9.807 X 263311935000
= 6.85924762e14 joules
H = 379.992032 X 0.75
= 284.994024m
M = 1.2830474e+14 X 2.83
= 3.63102414e14g
= 363102414000kg
E = 284.994024 X 9.807 X 363102414000
= 1.01484815e15 joules
Finally, we add them all together for our final result.
E = 1.56918708e16 + 3.65346739e14 + 4.12891246e14 + 1.20389049e15 + 6.85924762e14 + 1.01484815e15
= 1.93747722e16 joules
= 4.63068169216061154 megatons
Remember that this is just the passive energy outputted by Kaalgrontiid every second to keep the islands in the sky.
3. Dragonhold explodes
Next for the energy of Dragonhold exploding. We'lll add all of the volumes for each of the islands for our total volume.
V = 1.34434271e15 + 4.10134314e+13 + 4.20053459e+13 + 1.41235173e+14 + 9.30430867e+13 + 1.2830474e+14
= 1.78994449e15cm^3
= 1789944490000m^3
We'll go with a low end of violent fragmentation and a high end of pulverization.
(Low end)
E = 1789944490000 X 69000000
= 1.2350617e20 joules
= 29.518683078393880947 gigatons
(High end)
E = 1789944490000 X 200000000
= 3.57988898e20 joules
= 85.561400095602294869 gigatons
Nahfahlaar survives this explosion close to ground zero.
4. Abnur Tharn blocks the explosion
The cinematic version of this feat is even more impressive; Abnur Tharn has no preperation for the aeonstone exploding, and stops it after it explodes. When he gives out and the explosion continues, we see it exploding rapidly to where the other heroes are. Which means Abnur Tharn reacted to the explosion. TES is a multiverse, so both versions are canon.
Timeframe is 1.12 seconds. Above we had the distance from the heroes to the island at 8136.6m.
T = 8136.6m/1.12s
= 7264.82143/340.29
= Mach 21.3489125