Should you kill Blognark? Only by mutual agreement with the player.
You should discuss this with the player before doing so. What you're describing is not a "naturally" occurring death due to emergent gameplay based on player agency, which is how the rest of the PC's died. You're describing a planned death that is central to the plot. If you do not get approval from the player, I suspect they will be very displeased.
The other PC's died as a result of the players' own agency in the story in some fashion or another, as you've described. You should afford Blognark's player the same sense by giving that player agency over whether you kill off his character. In addition, if it is planned with that player, you can ensure that the scene does indeed play out dramatically, so that you can all but ensure it will come off as the pivotal, motivating scene you want it to be. I think any other course of action will sour the experience for all your players, especially the one who plays Blognark.
How can you kill Blognark for the best experience? As the consequence of a selfless, heroic, voluntary act.
The most rewarding and motivational solution I've experienced to the goal you are trying to achieve is to allow Blognark to commit a singularly selfless, heroic act (presuming his alignment is somewhere on the neutral or good spectrum) that only he among all the PC's is capable of truly fulfilling. It has to be voluntary on Blognark's part in order for it to feel rewarding to the players at the table, and it should not be an act that some random mook or NPC could have fulfilled instead or people will wish it had not been Blognark.
It can be an act that occurs during combat but need not be.
For example, Blognark could die in combat against the antagonist you are trying to establish, perhaps by ensuring that Blognark is the only character of a level capable of combating the overpowered antagonist at this time. Blognark can choose to save his allies to buy them time to get stronger and avenge his death in the future.
Instead, maybe everybody could escape narrowly from the antagonist and Blognark could use his tremendous strength (let's just say I'm guessing that attribute based on the name) to prevent the collapse of an archway just long enough for all his allies to pass beneath it and escape. Blognark himself is unable to get out from under the arch safely and has a few knowing final words with his allies before he gives in to the impending muscle failure and is buried in the rubble.