Mark Price is the price on the price source. All assets traded on DMEX have a price source defined. For the top assets like Bitcoin and Ethereum, the price source is Bitstamp.net. Therefore in this cases the MARK PRICE is the Bitstamp price.
The MARK PRICE is used to settle and liquidate positions.
Positions can only be liquidated if the MARK PRICE reaches the liquidation price. DMEX cannot manipulate the price internally to liquidate your position!