It's a group that packs so many top players/teams that one of the tournament favourites will get thrown out no matter what.
For example: group with cooller, cypher, rapha, and some_noob, where only 2 players continue to compete after group stage.
for this to happen, the top and high seed of each group would feel threatened by the mid and low seed.
For example @DHW11
Do Cypher & Spart1e feel threatened by dem0n and reload?
Do Cooller & Avek feel threatened by amai and p0ni?
Do Strenx & Killsen feel threatened by madix and ventz?
Do Rapha & Tox feel threatened by matr0x and ins?
Group with Avek and Cooller might be called "semi-death group", because of few factors: Cooller started practising literally few days ago, Avek doesn't seem to be in shape at all and then there's Amai, who could possibly turn his online-heroism into reality, at least some guys say he can :) Highly improbable, hence 'semi'.
with seeds u can still get a group of death scenario. Ok currently there aren't enough good players for that around. But imagine Zero4 and czm or dahang wouldve been there too. You could get someone like spart1e k1llsen or tox dropping into 3rd seed. This would then create a group where it's likely to see 3 players with big chances to go through.
To put it into perspective, the seedings in the next Euro footy cup could create a group that would include Spain, Germany, Portugal and France. The very definition of a group of death.
Group of death is when there are more players/teams that have a good chance of going through in first place than there are players/teams qualifying from the group and the top teams would be virtually a guaranteed to go through from any other group.