I play a P&P tabletop game and we have a small party of evils. Now Im a dark elf and we have a dark elven prince. Now the prince has been made the air of the dark elven throne by his father and he is one out of 15 brothers. Now we want to enhance his position as the air of the throne and I would like to hear if anyone has ideas that would be good to enhance his position. Kill the brothers!

Or even better: Make the brothers kill each other! By dark elves do you mean drow, or ordinary elves, but evil? In the first case, your DM really should re-read everything he has about drow elves... seriously. It is Dark elves. It is a made up world so things are adepted to his ideas. Dark elves are evil?! Well, the only dark elves I know of are (apart from drow elves) those from Warhammer, which ARE evil. Very evil. Excactly the ones from warhammer are the ones he uses in his world...got any good ideas ten? Im veyr open to suggestions Create a plot that will make the other brothers fight each other and die in the result. Why waste their deaths?

Give some of the brothers a detatchment of troops, and orders to die rather than be captured (see to it that the troops will aid your brothers should they lack the moral fortitude when the time comes), and send them off to raid a kingdom that has been an ancestral enemy. Then send runners to the rival kingdom, telling them that a rogue element in your city is sending a raiding party against them, the prince is not like his father and wants to establish diplomatic relations. The rival kingdom should take care of the brothers you sent on the raid.

Worst case scenario: they capture one of the brothers alive and find out that you sent him. They already didn't trust you, and that brother winds up as an exile in that city.

Best case scenario: All the brothers die and you can establish diplomatic relations. Appoint the other brothers, except maybe one or two as envoys. Once you have access to a good target, destroy the target and betray your new allies. At the least, the rival kingdom should execute the envoys in retribution . With that done, you should have earned the recognition of your people (removing a power base from the remaining brothers) and weakened an enemy. If you gained any territory in your venture, make the last brothers governers and instruct them to tax heavily and mistreat the population. When the people complain, you then say you gave no such order, you lower their taxes slightly and publicly execute the last brothers for betraying you. That way the conquered people like you too.

That's how you get rid of the brothers.
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Two wrongs don't make a right but three lefts do.