Thor, King of Lightning

When Thor, King of Lightning attacks, attacking creatures gain trample until end of turn.
When Thor, King of Lightning attacks, roll a d20.
1 - Thor gets +0/+0
2 -4 - It gets +1/+0
5 - 7 - It gets +2/+0
8 - 10 - It gets +3/+0
11 - 13 - It gets +4/+1
14 - 15 - It gets +5/+1
16 - 19 - It gets +6/+2
20 - It gets +8/+4